Shams Charania of The Athletic and Stadium reported the star forward has been ruled out for Game 5 of the Eastern Conference Semifinals.
The 25-year-old was limited to 11 minutes in the Bucks' Game 4 win because of a sprained ankle.
Things were looking bleak for Milwaukee when Giannis exited in the second quarter Sunday, but the team held on to prevail in overtime. For that to happen again, the Bucks will likely need another huge performance from Khris Middleton, who posted 36 points, eight rebounds and eight assists in Game 4.
Nobody will be under more pressure than head coach Mike Budenholzer, though. The Bucks have gone 116-39 in the regular season with him at the helm, but he has looked incapable of making necessary adjustments during the playoffs.
Budenholzer will have to draw up a game plan Tuesday that doesn't revolve around his best player, which won't be easy since the Bucks largely assembled their roster with Antetokounmpo in mind.
If the Bucks get bounced in Game 5, Antetokounmpo's absence might not be enough to buy Budenholzer another season in Milwaukee following what would be a bitterly disappointing end to the year.