Fan puts on Giannis shirt over Westbrook jersey after big block

These fans ain't loyal.

A spectator on hand for Monday's tilt between the Milwaukee Bucks and the Oklahoma City Thunder was wearing a Russell Westbrook jersey ... for half the game.

In the third quarter, Giannis Antetokounmpo soared to reject Westbrook's attempt at the rim, which prompted the boy to put on a Greek Freak tee over the OKC garb.

We're assuming he didn't switch back, as Russ struggled down the stretch and the Bucks came away with the 98-94 win.