"I put a bubble on that rim and the only person allowed to go get a rebound was him," Sampson said.
Grimes, a 6-foot-5 sophomore guard who transferred from Kansas, is multitalented; he's a proficient scorer, a strong defender and checks a lot of the boxes coaches look for. When Sampson looks at the stat sheet and sees a "4" under Grimes' rebound total, though, he wants that to nearly double.
Enter the rebounding drill.
"Every time somebody shoots and it's not me, I gotta go get the rebound," Grimes said.
How'd it go?
"He was fighting the whole team," Sampson said.
That willingness, not just to rebound but to improve every aspect of his game, is driving Grimes in a solid start to his second collegiate season, a year after an up-and-down freshman campaign with the Jayhawks. The Cougars (6-2), who host Oklahoma State on Sunday (3 p.m. ET, ESPN), are hopeful that the hometown product can help them return and perhaps surpass the heights they reached a year ago.
Through eight games, Grimes is Houston's leading scorer (16.9 points per game) and the team's best defender, according to Sampson. Notably, when Grimes was courted as a recruit by major programs across the country, the one 36 miles south of his high school (College Park in The Woodlands, Texas) wasn't one of them.
"We didn't recruit him at all," Sampson said.
Grimes, a five-star recruit who was the No. 8 overall player in the 2018 ESPN 100, had his pick of blue bloods. He signed with Kansas, a perennial national championship contender, in November 2017. At that point, Houston hadn't even been to an NCAA tournament in seven years and had been only once in the past 25. The sparkling new Fertitta Center wasn't yet complete and the program's renaissance not yet fully realized.
"Our program was at a different point back then," Sampson said. "We didn't recruit those kids."
Though he started all 36 games as a true freshman, Grimes didn't flourish in Lawrence quite like he'd hoped.
"Every situation isn't always the right fit for the person's particular skill set," his father, Marshall Grimes, said. "Coach [Bill] Self is a great coach, he's a Hall of Fame coach, most of the people there are really good people. It's just that sometimes [the] situation is not exactly the way you thought it was going to be."
After his freshman season, Quentin went through the pre-draft process, attended the NBA draft combine in Chicago, worked out for multiple teams and received valuable feedback. He said most of the projections he received had him going early to middle in the second round, so he withdrew his name from the draft.
And even though many may had assumed that Grimes would be a one-and-done, he and his family never operated with that assumption.
"I never really just like set myself on one-and-done," he said.
Grimes decided to return to school, but he found out quickly that school would not be Kansas. Self had filled his scholarship spot, and said in a statement in May that he initially anticipated Quentin staying in the draft. Grimes was left no choice but to enter his name in the transfer portal.
A pair of programs closer to home were contenders: Houston and Texas A&M. There were several factors that contributed to his choosing Houston.
For starters, the profile of Houston's program had risen significantly from the days that it wasn't a factor in Grimes' initial recruitment, having gone to back-to-back NCAA tournaments, opened the Fertitta Center and become a program in the national conversation.
"UH was already a solidified program on the rise," Grimes said.
He also knew a few of the current players who hailed from the Houston area, namely forwards Fabian White and Cedrick Alley Jr. On his visit, the Cougars welcomed Grimes with open arms.
"The guys really embraced him," Marshall Grimes said. "It was genuine ... He really liked the players."
And the presence of Sampson -- a proven coach who received high recommendations from an NBA MVP -- was crucial. While in Phoenix during the pre-draft process, Quentin had a chance to play pickup games with James Harden of the Houston Rockets, with whom Sampson was an assistant before taking the Cougars' job in 2014.
Grimes said Harden told him that Sampson gives his guards a lot of freedom and that "it's kind of a player's program."
"He's where I'm trying to get to," Quentin said of Harden. "He's an All-Star ... Just seeing one of those guys that Coach Sampson coached [say those things] really just stuck out to me."
It was a ripe opportunity for the Cougars, who needed a guard after the departure of Armoni Brooks. The team's top 3-point shooter last season entered the draft and didn't return to school, which surprised Sampson. Grimes' availability became timely.
Once Grimes chose Houston, there was one important task to settle: whether he would be able to obtain a waiver to play immediately.
This past offseason, the NCAA's Division I council adjusted the guidelines for transfer waivers. In cases where a player couldn't return to his previous team (like Grimes), the releasing school could provide a statement from the athletic director stating so, and explaining why the player was transferring. The player must also show that the situation was outside of his or her control.
It was in this instance, Sampson said, that Self, a longtime friend, was helpful.
"Bill's a big reason why Quentin got eligible," Sampson said, "the support he got from Kansas and then the way that that message was relayed to the NCAA."
Kansas made it clear that Grimes was in good standing -- he simply wouldn't have a spot. The biggest remaining hang-up was the length of the process. Grimes transferred to Houston in July, and the school filed for the waiver in mid-August but didn't get an answer from the NCAA until early October. And his first answer was a denial.
According to Lauren DuBois, Houston's senior associate athletics director for compliance, the NCAA said that even though it was proved that Grimes wasn't able to return to Kansas, that Grimes' transfer was "within his control," because he chose to go through the pre-draft process.
DuBois wrote a two-page letter on Grimes' behalf, appealing the decision. Grimes and his parents also penned statements that proved critical. DuBois' point was that this was the pre-draft process working as it was intended: Shouldn't an athlete, if he decides he's not ready to go pro, be able to come back and play another year in college without being penalized for it?
"He shouldn't be punished because he chose to avail himself of that opportunity, and at the end of the day, make the right decision and go back to school," DuBois said.
The NCAA listened. A week later, the committee that heard the appeal approved his waiver unanimously. Grimes, upon finding out, left practice covered in sweat, sprinted to DuBois' office, and gave her a hug.
"The smile on his face was humongous," DuBois said.
Sampson said he saw Grimes' spirits rise almost immediately. His game followed. He scored in double digits in each of his first three games, including a career-high 32 points in a November win over crosstown rival Rice. That night, he was 11-of-17 from the floor, 4-of-6 from 3, and had nine rebounds.
On Sunday at South Carolina, his outside shot was falling again as he hit 4 of 7 3-pointers for a team-high 24 points. More importantly to Sampson, he has become a lockdown defender.
"Quentin Grimes is our best defensive player," Sampson said. "We put him on the other team's best player ... That boy's got good feet."
Grimes said he is enjoying emphasizing that part of his game.
"I know I can play defense and kind of contain my man, but now I'm kind of taking it a little bit more personal," he said. "I kind of want to shut them down."
And there's still plenty of room to grow. His coaches and those close to him are excited about his progress and his potential. Grimes, meanwhile, is happy and thriving in his new home and feels validated in his decision.
"This is a high-major program," he said. "Right now, what they're doing with the guards and the player development, I think it's one of the top programs in the country ... It's right up there if not better."