With Boston mired in an extended losing funk, Stevens asked Sullinger to take his game to a higher level and cement himself as a leader of a Celtics team that isn't afraid to lean on him despite his age.
The 21-year-old Sullinger responded by putting up game highs with 24 points and 17 rebounds, but he could only watch as Evan Turner's contorted floater at the buzzer lifted the Philadelphia 76ers to a 95-94 triumph Wednesday night at TD Garden.
The Celtics (15-33) have lost four straight and 19 of their past 22. With Wednesday's loss, they flip-flopped positions with Philadelphia and assumed the third-worst record in the NBA.
Sullinger has been one of Boston's bright spots in a season filled with struggles, hammered home by his invitation to the league's annual rookie/sophomore mixer as part of All-Star festivities in New Orleans in mid-February.
Playing out of position given the Celtics' absence of a pure center, Sullinger is averaging 12.8 points and 7.8 rebounds over 26.6 minutes per game. With the team's struggles, he is the only regular who remains in the positive in plus/minus at plus-7. It's a staggering stat considering that Boston is minus-206 as a team and the other most frequent frontcourt starter, Brandon Bass, is minus-240 for the season.
What's more, Sullinger is playing with both a bruised hand and a dislocated index finger on his shooting hand -- an injury that popped out of place again during Wednesday's loss.
Stevens wouldn't demand more out of Sullinger if he didn't know he was capable of it. The Celtics desire more consistency at both ends of the floor, something that will only help Sullinger blossom into the superstar they seem to believe he can become.
"First of all, I did say, 'Hey, I realize you're playing through the hand and the finger,' because he's a tough guy, he wants to play, he likes to play and I value that, and I think highly of him for that," Stevens said. "And one of the things that I just challenged him on is not having to accept being a 22-year-old in the league.
"We are in a unique situation in that we're asking some of our young guys to be almost leaders, and almost more vocal in their approach. My talking point with him was, 'I know you're 22, but you're a mature basketball player, you know the game, and for our team to grow we need for you to maybe play and be a few years ahead of where you are.' And it's not fair to him, but it's a great opportunity for him, so that was my challenge to him."
Sullinger, actually, doesn't turn 22 until March 4. Nonetheless, the challenge issued to him secures his spot as one of the blocks of the Celtics' foundation during this transition process. There is, of course, the chance that the team could use him and his bargain-basement rookie contract as trade bait in order to lure a superstar veteran -- Sullinger could play the role of Al Jefferson in a Kevin Garnett-like deal. But Boston would much prefer to keep the young big man and use its other assets to fill in around him.
Sullinger's emergence is remarkable, particularly when you look at the others participating in the Rising Stars game. Here are the draft positions for the rest of the invited sophomores: New Orleans' Anthony Davis (1st), Washington's Bradley Beal (3rd), Cleveland's Dion Waiters (4th), Toronto's Jonas Valanciunas (5th), Portland's Damian Lillard (6th), Golden State's Harrison Barnes (7th), Detroit's Andre Drummond (9th) and Houston's Terrence Jones (18th).
The Celtics watched Sullinger fall to them at No. 21 in the 2012 draft. His rookie season was cut short by the back concerns that allowed him to slide out of the lottery, but Sullinger diligently navigated his rehab to be back on the floor for the start of this season.
Thursday, in fact, marks the one-year anniversary of the game that ended his rookie campaign when his lower back flared in the opening minutes of a game against the Sacrament Kings. Sullinger went under the knife soon after to alleviate the discomfort.
Even after a somewhat shortened offseason, Sullinger's scoring average has more than doubled since his rookie campaign, though his field goal percentage is down as he incorporates a 3-point shot into his arsenal. His offensive game as a whole has been a mixed bag lately, hindered in part by his hand maladies. During Wednesday's loss, Sullinger finished 9-of-25 shooting and lamented missing "a lot of shots that I normally make."
On the other end of the court, the Celtics own a defensive rating of 101.4 points per 100 possessions when Sullinger is on the floor. That's two points better than the team's season average, and more than four points better than when he's off the court. Even still, the Celtics would like to see Sullinger improve as an individual defender, becoming a more vocal presence, despite being routinely asked to go up against players bigger than him.
More than anything, it comes down to finding the consistency that veteran players display more often than their younger counterparts. Stevens knows the potential that Sullinger can play to at both ends of the floor; now he wants him to strive to maintain those levels and not use his age as a crutch when he fails to do so.
"It's a lot less about the numbers or how you play or how many shots you make or how many rebounds you get and more about, hey, a lot of young guys accept going through ups and downs because they think it's part of the process, and you can miss opportunities if you do that," Stevens said. "Our guys have great opportunities and I don't want them to miss it."
For his part, Sullinger acknowledged Stevens' challenge and pledged to play beyond his years. On a night the Celtics were without Rajon Rondo and had Jeff Green disappear after the first quarter, it was Sullinger who shouldered much of the load and put the team in position to get a much-needed win.
But wins and losses won't determine how Sullinger fares in Stevens' challenge. Sullinger played beyond his years on Wednesday and, if he does that consistently, Boston should win more games than it loses down the road.