Celtics will have to make up for the loss of Marcus Smart

The starting guard will be sidelined for at least four weeks with a partial tear of his left oblique muscle.

BOSTON – He was unable to start the 2018 NBA playoffs on time because of a torn ligament in the right thumb.

Marcus Smart was able to return for Game 5 of the opening-round series against the Milwaukee Bucks and helped the Celtics get within one win of a trip to the NBA Finals.

One year later, Smart will again be a spectator when the playoffs get going, only this time he won’t be returning at any point in the first round.

Smart has a partially torn left oblique muscle that will sideline him for at least a month and up to six weeks, meaning he might need the Celtics to get back to the conference finals in mid-May to play again this season.

The Celtics get the opening round going on Sunday at 1 p.m. against the Indiana Pacers at the TD Garden, and coach Brad Stevens will have to replace Smart in the starting lineup. Jaylen Brown is probably going to get the call to return to the starting role after being replaced in late November.

Smart was hurt last Sunday night in the home regular-season finale when he collided with Orlando Magic center Nikola Vucevic during the third quarter. He tried staying in the game after a timeout but collapsed to the floor in pain a short time later with an MRI done on Wednesday revealing the partial tear.

The Celtics, who went through last season’s playoffs without Kyrie Irving and Gordon Hayward, are going to be without one of their top defenders in Smart, who had improved his shooting this year.

“It obviously stinks for him,’’ said Stevens before practice at the Auerbach Center Thursday afternoon. “He’s a big part of our team and we’ll have to pick up what he does collectively. Obviously he’s unique in the way that he plays. To get the news (Wednesday) was a downer for him.’’

Smart, who signed a four-year, $52-million contract last summer, put together one of his best seasons, becoming more reliable as a shooter while providing the Celtics with his usual tough defense to go with an ability to make clutch plays.

“I feel really bad for Marcus,’’ said president of basketball operations Danny Ainge. “I know how well he’s playing right now. I know how hard he’s worked to get ready for this moment. But injuries are just part of what we all have to deal with and it’s an opportunity for somebody else. Hopefully we can keep playing so Marcus gets a crack at the playoffs this year.

“He was really ready. He’s had a terrific season and I think he’s even been better lately. We need his defensive presence. Last year we didn’t have Kyrie and Gordon’s offense, and this year we don’t have Marcus’ defense.’’

Stevens said he will make a final decision on who starts in Smart’s place after the practices are complete this week, but Brown and Terry Rozier would be the first two in line, with Hayward a possibility.

“You go through every piece of information with regard to what group plays best together, who matches up well with Indiana, both starting and off the bench,’’ said Stevens.

Brown put together a strong finish to the regular season after struggling early while Rozier had a disappointing regular season after a solid showing in the playoffs last spring when he replaced Irving.

“Obviously it’s tough, especially being a guy like Marcus with his presence and the way he changes the game. It’s tough seeing that,’’ said Rozier. “I feel like I’m more important to the series now, I’m more needed. Obviously I’m going to have to step up and I’ll be ready for it. I’m just ready for the opportunity. It’s me stepping up and being ready when my name is called.’’

If Smart misses four weeks, he would be back around May 15, which is around the time that the Eastern Conference finals are scheduled to begin.

“Four (weeks) is pretty aggressive, from what I’ve been told,’’ said Stevens. “We’ll see what happens.

“I feel for Marcus. He’s a competitor and he loves this time of year.’’

But for the second year in a row, Smart will be watching from the bench as the playoffs get going this weekend. And unlike last season, there will be no first-round return for him.

“He was bummed,’’ said Ainge. “He was in pain. Thank goodness we have other guys who can step up and take Marcus’ spot. No one completely replaces Marcus, but it’s an opportunity for other guys to play.’’