"I think it was mainly what I did to myself," Simmons said after failing to score a field goal for the first time in his 88-game career. "Mentally, I was thinking too much, overthinking the plays. Wasn't just out there, flowing, playing the way that I play, which is free.
"I think, obviously, [the Celtics] have a game plan, I know what their game plan is, and I've got to play my game."
Boston -- which has a 2-0 lead in the best-of-seven series -- took a physical approach in defending Simmons. The Celtics forced Simmons to turn the ball over five times and contained his drives, limiting him to only one point on seven drives to the basket, according to ESPN Stats & Information.
"It took everybody," Boston's Marcus Smart said. "We threw different matchups at him, and we really dictated what he wanted to do."
Simmons, though, said the Celtics' physical play didn't bother him. Instead, he said his struggles were "self-inflicted."
"Mental, thinking too much," the Rookie of the Year candidate said after setting a career low for points. "I think the way the Heat played [against Simmons in the first round], it was nothing compared to the Celtics. Physically, the Heat were on another level."
Simmons was fantastic in the Sixers' first-round series against Miami. In five games, the 6-foot-10 point guard averaged 18.2 points, 10.6 rebounds and 9.0 assists. He had 18 points, 7 rebounds and 6 assists in the Sixers' Game 1 loss to Boston but was shut down Thursday.
"I'm going to have bad games; it happens," Simmons said. "Obviously, it's not the perfect timing. But we're headed home now."
Simmons' struggles in Game 2 were in stark contrast to his backup, T.J. McConnell. McConnell had eight points, five assists and two steals in 17 minutes. The Sixers outscored Boston by 16 points with McConnell on the floor.
McConnnell's performance -- particularly in the fourth quarter -- left Sixers coach Brett Brown with a difficult decision.
He could have left McConnell in and kept his star rookie on the bench. But Brown chose to sub in Simmons for McConnell with 5:29 to play after McConnell had four points and three assists in the period.
Philadelphia was up by two when Simmons checked in. The club ended up losing by five, getting outscored 17-10 to close the game.
"It's a tough decision, I admit it," Brown said of subbing in Simmons for McConnell in the fourth quarter. "This whole playoff experience is something that I want our young guys, our star players, to learn from and grow. The decision -- do you go with T.J. still, or do you go back to Ben Simmons -- I'm coming back to Ben Simmons. I'm coming back to Ben.
"He's had a helluva year, I think he's the Rookie of the Year. I think he's going to have to learn to play in these environments. And I'm going back with Ben Simmons."
History is not on the Sixers' side as they head home for Game 3 in Philadelphia on Saturday. Entering this postseason, teams that lost the first two games of a best-of-seven series on the road have won only 6 percent of those series (15-236), according to the NBA.
Despite those long odds, Brown, Simmons and Joel Embiid each said that the Sixers wouldn't let Thursday's loss impact them as they prepare for Game 3.
"We're learning, we're a young team. Nobody is panicking in the locker room, nobody's freaking out," Simmons said. "We're staying together as a team. We go home, and we've got to take care of business."