BOSTON — The Celtics say forward Gordon Hayward will be sidelined for around four weeks after suffering a severe right ankle sprain in Boston’s Game 1 win over Philadelphia in their first-round Eastern Conference playoff series.

The injury occurred in the fourth quarter of the Celtics’ 109-101 victory Monday night when he went up for a rebound and rolled his ankle as he landed on teammate Daniel Theis’ foot. He limped to the locker room and didn’t return.

Hayward played 34 minutes and scored 12 points on 5-for-13 shooting with four rebounds before the injury, which the team described as a Grade 3 sprain, the most severe type.

The timeline for recovery means Hayward will miss the rest of the first round at a minimum and possibly some of the second if the Celtics advance. He already planned to leave the bubble for a time if the team was still playing in September. His wife is pregnant and is scheduled to deliver sometime next month.

Jayson Tatum had career playoff highs of 32 points and 13 rebounds, and Boston beat Philadelphia 109-101 in Game 1. With Hayward out, it could mean a shift to a three-guard lineup and more minutes for Marcus Smart, who has been filling the role of Boston’s sixth man. Smart played 32 minutes in Game 1. He started 40 games during the shortened regular season.

The Celtics went 14-6 without Hayward this season.