BOSTON (CBS) — The Celtics are going to be without Gordon Hayward for at least a month. The Boston forward has been diagnosed with a Grade III sprain of his right ankle, and is expected to miss four weeks, the team announced Tuesday.

Hayward was injured late in Boston’s Game 1 victory on Monday night, rolling his ankle when he came down on teammate Daniel Theis’ foot while trying to pull down a rebound over 76ers center Joel Embiid. He fell to the floor in obvious pain and had to be helped to the Boston bench. He walked to the Celtics locker room on his own power, but was wearing a boot and walked on crutches as he left the arena.

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Grade III sprains are a complete tear of affected ligaments, so it was a pretty severe injury that Hayward suffered on Monday night.

Given his timeline for a return, which appears to be on the optimistic side, Hayward could maybe be back on the floor if the Celtics make it to the Eastern Conference Finals, which are set to begin Sept. 15. But that is around the time he is expected to leave the NBA bubble for the birth of his son, so he could be out even longer.

This is a big loss for the Celtics, as Hayward had his best year in Boston during the regular season, averaging 17.5 points, 6.7 rebounds and 4.1 assists per game. He shot 50 percent from the floor and 38 percent from three-point range. He was averaging 18.7 points and a team-high 7.7 rebounds in NBA bubble action.

Hayward did a little bit of everything over his 34 minutes of action in Game 1, scoring 12 points off 5-for-13 shooting while adding four rebounds, three assists and four steals before getting injured.

Boston has survived without Hayward before, including this season when he missed a month of action after breaking his hand in November. Boston went 14-6 in the 20 games that Hayward missed, but he has become a consistent and important part of the team’s offense. The 76ers proved to be a pesky bunch on Monday night, and losing Hayward will make it all more difficult for the Celtics.

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With Hayward now sidelined, Marcus Smart will likely take over in the starting lineup. Game 2 against the 76ers is set for Wednesday night.