BOSTON (AP) The youth movement is alive and thriving for the Boston Celtics.

And for the second straight game it was enough to upend a Philadelphia 76ers team that still has much to figure out as it heads home trailing 2-0 in the Eastern Conference semifinals.

Rookie Jayson Tatum scored 21 points and hit a pair of free throws in the closing seconds, helping the Celtics rally from a 22-point deficit to beat the 76ers 108-103 on Thursday night.

Terry Rozier added 20 points, nine assists and seven rebounds. Marcus Smart finished with 19 points and five rebounds as the Celtics improved to 6-0 at TD Garden this postseason. They have never blown a 2-0 lead.

''Coach was saying whoever is the tougher team is going to win,'' Rozier said. ''They definitely punched us in the mouth at the beginning of the game. We just had to bounce back and get things together.''

Game 3 is Saturday in Philadelphia.

J.J. Redick had 23 points for the 76ers, making five 3-pointers. Robert Covington added 22 points and nine rebounds. Joel Embiid finished with 20 points, 14 rebounds and five assists, but rookie star Ben Simmons missed all four shots and had just one point.

''Mentally, just thinking too much,'' Simmons said of his tough night. ''I'm going to have bad games. It happens. Obviously, it's bad timing.''

Philadelphia recovered after squandering its big lead to nudge back in front 93-88 midway through the fourth quarter.

But an 11-4 run put Boston back in the lead 99-95 with less than four minutes to play.

It was 101-97 when a missed Philadelphia jumper led to a 2-on-1 fast break and alley-oop dunk from Rozier to Tatum with 2:23 left.

A layup by Dario Saric cut it to 104-101. But Al Horford was able to drive past Embiid for a layup with 8.3 seconds left.

Saric scored out of a timeout to make it 106-103. The Sixers quickly fouled Tatum, who calmly hit a pair of free throws.

Following an anemic offensive effort in Game 1, the Sixers looked rejuvenated early in Game 2, using a 14-2 run to begin the second quarter on their way to building a 48-26 lead.

It didn't last.

The Celtics responded by ending the half on a 25-8 run. It included three straight 3-pointers and a tip-dunk by Jaylen Brown, who returned to action after sitting out the series opener with a strained right hamstring. He came off the bench and played 25 minutes, scoring 13 points and grabbing four rebounds.

The Sixers' lead was completely erased midway through the third quarter, when Aron Baynes got a 3-pointer to bounce in and put Boston in front for the first time.

The lead grew to 76-68 on a baseline dunk by Tatum that capped a 50-20 Celtics run.

''Once we got on that run we never looked back,'' Rozier said.

Brown entered the game for the first time at the 7:14 mark of first quarter. It took less than a minute to make an impact. He missed his first shot attempt before chasing down a loose ball and sprinting ahead for a one-handed dunk .

Prior to Thursday, Brown had started every game he'd appeared in this season. It was his first time off the bench since Game 5 of last season's Eastern Conference Finals.

''They won two games at home and we have to reciprocate,'' Sixers coach Brett Brown said. ''There's not going to be a pity party.''

TIP-INS

76ers: Outrebounded the Celtics 49-41. ... Went 13 of 33 from the 3-point line.

Celtics: The 22-point comeback victory represents their largest come-from-behind playoff victory since Game 4 of the 2008 NBA Finals (24 pts). ... Went 15 of 36 from the 3-point line.

HE SAID IT

''It's like a movie. You only see these type of comebacks in movies.'' - Celtics guard Marcus Smart.

SLOW START

Despite Brown's early highlight, the Celtics were sluggish early.

They started off the game 0 for 5 and didn't score until Smart's 3-pointer at the 8:37 mark of the first quarter.

Horford (1 for 3), Rozier (0 for 4), Brown (1 for 4) and Marcus Morris (0 for 3) were a combined 2 for 14 in the period, allowing Philadelphia to build a 31-22.

CELEB SIGHTINGS

Philadelphia-born rapper Meek Mill sat courtside for Game 2. Mill was released from jail last week after spending nearly five months behind bars on a probation violation. Rapper Gucci Mane sat across from the 76ers bench wearing a Brown jersey. The two met last year when Brown attended one of his concerts.

Another Philly native, actor Kevin Hart, sat a few seats down from Gucci. Hart at times antagonized the TD Garden fans, drawing boos.

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