MIAMI (AP) -- Giannis Antetokounmpo, the league's leading scorer this season, was unexpectedly held out of the Milwaukee Bucks' NBA Cup game at the Miami Heat on Tuesday night with swelling in his left knee.

He had been expected to play until about an hour before the game. The team had Antetokounmpo listed as probable with a strained left calf, and then the knee issue evidently flared up.

"I never know," Bucks coach Doc Rivers said, about 90 minutes before game time and moments after the team said there was an issue with the knee. "Honestly, that's breaking news to me, too, right now."

Milwaukee doesn't play again until hosting Washington on Saturday. The NBA -- under the player participation policy that went into effect last season -- has a rule stating that "unless a team demonstrates an approved reason for a star player not to participate in a game," it must have the star players "for all national television and NBA In-Season Tournament games."

Antetokounmpo, a two-time MVP, obviously qualifies as a star player by league definition, and the game in Miami was both nationally televised on TNT and an NBA Cup game. The league fined the Atlanta Hawks $100,000 earlier Tuesday for violating the policy by holding Trae Young out of a Cup game against Boston on Nov. 12, after a probe concluded he could have played.

Antetokounmpo -- the reigning Eastern Conference player of the week, an award he has captured 24 times -- not playing against the Heat does not necessarily mean there will even be a league investigation. The eight-time All-Star is averaging a career-best 32.4 points on 61% shooting this season, and he played in 16 of Milwaukee's first 17 games.

"Listen, the way he plays and how he works, there's going to be things like this," Rivers said. "And when they come, you just deal with them."

Khris Middleton, Milwaukee's three-time All-Star forward who has yet to play this season after undergoing surgery on both ankles in the offseason, is ramping up toward a return by getting some 5-on-5 work in on the practice floor.

But there doesn't seem to be a timetable for a return to games.

"He's not ready to play yet, but he's working hard," Rivers said Tuesday. "He's gone through 5-on-5. We've done some of that. The next steps are getting on the floor, but I can't tell you when that's going to happen."

Rivers noted that Middleton has been playing 4-on-4 for some time and cautioned against reading too much into 5-on-5 work.

"I don't know what the difference is," Rivers said. "We added a guy, so I wouldn't make that big of a deal about it. The key is we just try to get him back on the floor. He's got to go through the process."