BOSTON — The Boston Celtics said Wednesday that guard Marcus Smart will be sidelined 4 to 6 weeks after being diagnosed with a partial tear to his left oblique abdominal muscle.

The injury was revealed in an MRI scan Wednesday. Smart sustained the injury during the third quarter of Boston’s home loss to Orlando on Sunday. He was helped off the floor by members of the training staff and was holding his left side after colliding with Magic center Nikola Vucevic.

Boston is set to face Indiana in the first round of the NBA playoffs. The timetable for Smart’s return means he will be sidelined for that series and the second round as well, should the Celtics advance.

It’s not known who will replace Smart in the starting lineup, although Jaylen Brown played  extensively in the postseason a year ago. Last year the Celtics reached Game 7 of the conference finals while playing without Kyrie Irving and Gordon Hayward, who were both recovering from injuries.

Last season, Smart returned from a thumb surgery late in the first round of the playoffs despite initially being given a recovery window of 6 to 8 weeks.

Also, forward Jayson Tatum said he feels “way better” as he looks to return from the shin contusion that took him out of the game against Orlando Magic on Friday.

Will he be ready for the postseason, which is approaching this weekend?

“Hope so,” Tatum said. “At this rate, (I’m) feeling a lot better. Like I said, I practiced, I went through a lot of reps today. I’ll practice tomorrow, get some rest.”

The Celtics are scheduled to practice again Thursday.

Tatum said he had developed soreness on his shin before bumping his knee on something that aggravated it. He felt good going through warmups and tried to play, but the injury started bothering him during the game and he was removed.

“(I’m) not trying to rush anything,” Tatum said. “I usually play. I hate sitting out games. I try to play every game of the season but it was really bothering me. I knew that if I took myself out the game I was really in pain.”

Coach Brad Stevens said Tatum didn’t go through anything live during practice but looked good in his limited action.

The Celtics, as the fourth seed in the Eastern Conference, will have home court in the best-of-seven series against the Pacers, who are the fifth seed.