Nathaniel S. Butler/Getty Images

Boston Celtics guard Marcus Smart is suffering from calf tightness and considered out indefinitely as the United States prepares for the 2019 FIBA World Cup, according to the New York Times' Marc Stein. 

Stein added Smart will be re-evaluated next week.

Team USA opened its training camp Monday in Las Vegas.

Smart's injury adds to a number of notable absences for the national team. According to NBA.com's John Schuhmann, 35 players were listed on the roster for Team USA's 2018 minicamp before a large number dropped out. Five holdovers from the original roster remain.

"I don't even worry about that," head coach Gregg Popovich told reporters regarding the turnover. "I only care about who is here. I've got a fine group of guys and we are going to try and get them to form the best possible team we can."

In terms of backcourt options, the United States still has Kyle Lowry, Donovan Mitchell and Kemba Walker.

The fact Smart will miss the rest of training camp could jeopardize his hopes of making the final 12-man squad for the World Cup. Of course, that would leave Popovich with only three natural guards.

The U.S. opens the tournament Sept. 1 against the Czech Republic. If Popovich and the Team USA training staff are confident Smart will be healthy by then, taking him to China would make sense even with his lack of practice time in the buildup to the event.