Tatum will not play against Japan on Thursday or in the Americans' first second-round game on Saturday. He will be re-evaluated Monday, the same day the Americans play their final second-round matchup.
Tatum - one of many heroes for the U.S. in its dramatic 93-92 overtime win over Turkey on Tuesday - got hurt on the Americans' final possession of the game, twisting his ankle on a drive into the lane. Tests performed Wednesday confirmed the sprain.
There's not much at stake for the U.S. against Japan. The Americans have already clinched the top seed out of Group E and are assured of playing a pair of second-round games in Shenzhen, China, on Saturday and Monday. If the Americans keep advancing, the quarterfinals would be on Sept. 11, semifinals on Sept. 13 and the title game on Sept. 15.
Tatum has started each of the Americans' two games so far at the World Cup.
He was fouled while taking a 3-pointer with one-tenth of a second in regulation and the U.S. down by two on Tuesday. Tatum made the first free throw, missed the second and made the third to tie the game and force overtime.
''I wasn't nervous,'' Tatum said of being in that spot. ''I'm still (mad) I missed the second one. Then I wouldn't have got hurt. We won. Glad I was able to help give us some extra time and extend the game. As long as we win, that's all that I care about.''
He had more heroics in overtime, corralling a loose ball off a missed Turkey free throw with about 8 seconds left and the U.S. trailing by one. He dribbled downcourt, found Khris Middleton with a bounce pass that allowed the Milwaukee forward to get to the rim and get fouled. Tatum got hurt shortly after making that pass, falling in the lane.
''I think I just slipped and fell the wrong way,'' Tatum said.
Middleton made both free throws, and the U.S. escaped.
Celtics coach Brad Stevens, speaking to reporters in Boston a couple hours after Tatum got hurt, said it didn't sound like the injury was serious. Boston has four players on the U.S. roster and six players at the World Cup overall, and Stevens said he had no reservations about them participating and risking injury.
''At the end of the day, they're going to be playing 5-on-5 other places,'' Stevens said. ''I don't think you can get any better prep than doing what those guys are doing, playing in environments where everybody is rooting against them, tight games.''
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