Boston Celtics guard Marcus Smart has suffered a torn tendon in his right thumb and will be out indefinitely, league sources tell Yahoo Sports.

The loss of Smart could be potentially devastating for the Celtics. He is expected to receive a second opinion on the thumb, which he injured diving for a loose ball in Sunday’s loss to the Indiana Pacers. Smart posted 20 points, eight assists, seven rebounds, four steals and six 3-pointers in 36 minutes on Sunday.

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The Celtics have gone 6-2 since Smart returned from a right-hand injury that forced him to miss one month. Smart has served as the Celtics’ top defender and emotional leader, and is a developing offensive player. He has averaged 10.2 points, 4.8 assists and 3.5 rebounds in 29.9 minutes per game this season.

Boston (46-21) currently has a firm grip on the second seed in the Eastern Conference, but also lost center Daniel Theis for the season because of a torn meniscus in his left knee. The Celtics are expected to be without All-Star guard Kyrie Irving and forward Jaylen Brown through the week as well. Boston remains without Gordon Hayward because of a fractured leg and ankle injury suffered in the season opener in October.

Smart, a 6-foot-4 guard able to play both backcourt positions, has spent his entire four-year career with the Celtics. He was the No. 6 overall pick in the 2014 NBA draft.

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