Toronto trailed 75-66 with 4:31 left in the fourth when Garbajosa went up for a blocked shot on Jefferson. Garbajosa fell awkwardly on his left leg and screamed in pain.
Garbajosa, who had five points and five rebounds, left on a stretcher and was taken to a hospital. Toronto spokesman Rover Yau said Garbajosa would stay in the hospital overnight for evaluation.
Toronto should know the extent of the injury on Tuesday. One thing's for sure, it wasn't pretty.
"I really thought he broke his leg," Jefferson said. "We bumped, my leg hit his leg, and I looked down and he was all twisted up. He was screaming, and I could feel his pain."
Garbajosa, who's averaging 8.5 points and 4.9 rebounds, has started 60 games. In Toronto's 121-94 win over Denver on Friday, he scored a career-high 22 points.
"He's a big part of our team, so we're definitely going to miss him," guard T.J. Ford said. "I hate that it happened because he brings so much to the table. On the defensive end, on the offensive end, his knowledge of the game -- man it's tough."
Ford scored 28 points to lead Toronto and Anthony Parker added 16 points.
It's the second big loss for Toronto in less than a week. Rookie forward Andrea Bargnani had an appendectomy last Wednesday and is expected to miss the remainder of the season.
Toronto has overcome several injuries this year, yet finds itself chasing its first division title in franchise history and first playoff berth since 2001-02. The Raptors are six games ahead of New Jersey in the Atlantic Division with 12 games to go.
Chris Bosh and Ford missed considerable time earlier this year, Jose Calderon has struggled with a sprained left ankle and Parker just returned to the lineup after missing eight of nine games with a sprained left ankle.
"That hurts us, but the team has gotten it done without me, it's gotten it done without T.J., Jose, Anthony -- the list goes on," Bosh said. "This is just another situation that we have to get ready for and make sure everybody steps up their play."
Even before Garbajosa's injury, Toronto struggled to keep pace with Boston.
Bosh had an off night, scoring 15 on 5-of-19 shooting. He also had 11 rebounds while being guarded by Kendrick Perkins.
"I think (Perkins) had Bosh frustrated," Jefferson said. "I mean, Bosh really saw a lot of shots that I know he didn't make. A couple shots he did make, he had to make a tough shot."
Delonte West scored 18 points and Rajon Rondo had 16 points and eight assists for the Celtics, who trailed only in the opening minutes and became the last team in the NBA with 10 home wins. Boston is a league-worst 10-25 at home.
The Raptors used an 8-2 run to cut the deficit to 80-74 with 2:59 left in the quarter, but Jefferson's layup extended Boston's lead the next time down the floor and helped Boston pull away.
The Raptors dropped to 3-1 against the Celtics, missing a chance for their first season sweep of Boston.
"These guys are making the plays necessary and playing tough defense," Pierce said. "I think we have to give credit to our defense. We didn't shoot the ball particularly well, but we got the turnovers and got the easy baskets. That's all you need in games like this."
Boston used a 13-4 run spanning the second and third quarters to take control.
After a pair of free throws by Joey Graham cut Toronto's deficit to 40-38 with five second left in the half, Rondo drove the length of the floor and hit a 19-footer at the buzzer to give Boston a 42-38 lead.
The Celtics scored the first five points of the third.
"Nobody played good," Toronto coach Sam Mitchell said. "We didn't play the way we've been playing, we threw the ball away, we tried to make great passes instead of make simple passes."
G Luke Jackson didn't play for Toronto after signing a 10-day contract on Sunday. Jackson was waived by the Celtics during the preseason. ... Boston C Michael Olowokandi will miss the rest of the season with a broken bone in his left foot. Olowokandi averaged 1.7 points and 2.0 rebounds per game in 24 games. ... Boston outrebounded Toronto 45-35.