BOSTON — The 76ers are heading to the Wells Fargo Center with a difficult challenge.

They fell behind two-games-to-none in the Eastern Conference semifinal to the Boston Celtics after Thursday's 108-103 Game 2 loss at TD Garden. The Celtics have never lost a best-of-seven playoff series after winning the first two games. They are 36-0 in that scenario.

On this night, the Sixers squad that couldn't hold a lead for most of the regular season resurfaced. This time, they blew a 22-point second-quarter advantage en route to the setback.

However, they still had a chance late in the game.

Dario Saric scored on a layup with 5 seconds left to close the gap to 106-103.  Boston's Jayson Tatum, who led the Celtics with 21 points, was fouled on the ensuing possession, coming out of a timeout. He made both foul shots with 3.4 seconds left to give Boston the five-point victory.

"I think we are going to be fine," Joel Embiid said of the series. "We feel like, especially tonight, we started off really well then we kind of lost it. So we know what works and what doesn't. We got to go back and watch film and learn from it."

Ben Simmons definitely needs to learn from his performance.  The Sixers point guard had his worst offensive performance of the postseason.  The rookie-of-the-year frontrunner had one point, five rebounds, seven assists and five turnovers.  He attempted just four shots — all misses.

The Celtics did a great job defending him. Philly native Marcus Morris provided solid man-to-man defense, which gave Simmons a tough time penetrating. On the occasions that he did go toward the rim, the Celtics had a wall of defenders preventing him to get close to the basket.

Simmons thinks his lackluster performance was more of a mental thing than Boston's defense.

"Mentally, I was thinking too much, overthinking the plays," he said, "and I wasn't out there flowing and playing the way I play, which is free."

Simmons said he's aware of the Celtics' game plan of being physical. The Australian believes he just has to play his game.

But the 6-foot-10, 240-pounder rarely has games where he "thinks too much."

"That's the thing. I'm going to have bad games," he said. "It happens. It's not the [most] perfect timing.

"You held home. We've got two home games we've got to take care of and handle business."

JJ Redick had a game-high 23 points while making 5 of 9 three-pointers. Joel Embiid had 20 points and 14 rebounds. Robert Covington erased memories of scoring just three points Monday while missing all six of his shots. He had 22 points, shooting 8-for-15, including 4-of-7 on three-pointers.

Boston's backcourt did most of the damage with Terry Rozier scoring 20 points and Marcus Smart adding a huge 19.

Sixers backup point guard T.J. McConnell was a spark off the bench. He scored eight points on 4-for-4 shooting and had five assists while playing with grit. He was also a team-high plus-16.

The game marked the return of Boston's Jaylen Brown.  The shooting guard had been listed as doubtful on Wednesday. He scored 13 points in 24 minutes, 41 seconds off the bench.

He suffered a right hamstring injury during Saturday's Game 7 first-round victory against the Milwaukee Bucks. He missed the entire second half of that game and sat out Monday's game against the Sixers.

The Celtics took their first lead of the game on Aron Baynes' three-pointer with 7:45 left in third quarter. Brown's three-pointer gave them a 72-66 advantage with  3:58 remaining in the quarter, and led 76-68 after Tatum's dunk at 2:24.

The Sixers knotted the score at 79 just 47 seconds into fourth after consecutive baskets by McConnell. Then he assisted on a three-pointer by Covington to give Philly an 82-81 lead with 9:46 remaining.

After a 5-0 run by Boston, the Sixers scored six straight points on three-pointers by Redick and Embiid to take an 88-86 lead with 7:42 to play.  The Celtics knotted the score before Covington's three-pointer at the 7:17 mark made it 91-88. The lead increased to 93-88 on an Ersan Ilyasova lay-up with McConnell picking up the assist.

But Boston responded and Morris gave the Celtics a 96-95 lead with 4:16 to play. Terry Rozier's corner three extended the lead to four 40 seconds later.

The Celtics' lead reached 103-97 after Tatum's alley-oop dunk from Rozier with 2:23 left.

The Sixers got off to a fast start thanks to Redick, who scored the first eight points en route to scoring 13 on 5-of-6 shooting in the opening quarter.  Embiid grabbed six rebounds to go with two points and Marco Belinelli added six points (the only six points he would score) as the Sixers enjoyed a 31-22 lead.

The Sixers extended their cushion to 22 points (48-26) on Covington's three-pointer with 6:41 left in the half. Philly would lead 51-31 after another Covington three-pointer with 5:22 remaining before intermission. But the Sixers were outscored, 20-5, the rest of the half as they clinged to a 56-51 halftime advantage. Trying to let the players figure things out, the Sixers didn't call a timeout while things were unraveling at the end of the half. That might have led to them losing the momentum they had produced.

On this night, McConnell was more effective than Simmons. However, the Sixers took the reserve out of the game with the team up, 93-91, with 5:29 remaining.

"I play until coach takes me out," McConnell said. "It's not a big deal he went with Ben. Ben's gotten us to this point.

"Coach is the reason we won 52 games. Ben is the reason we won 52 games. I think he's a borderline all-star as a rookie. Who wouldn't go with him?"