You can stop him, it's actually fairly simple. You pack the defense in and collapse when he drives, so there are multiple defenders there no matter which way he goes. The problem is that this leaves the shooters open at the 3 point line. Milwaukee may not have great 3 point numbers, but if you let Middleton, Brogdon, or Terry set up uncovered on the 3 point line they are going to make you pay.
So you try to mix it up, sometimes you collapse when he drives, and try to recover when he passes out of the paint, other times you play him straight up and hope the defender can get it done. I'm actually not too concerned with Giannis, he's going to get his points. The Celts want to make him work hard, mix up defensive looks, keep him off balance, try to get him to take jump shots, but don't worry too much if scores 28 +.
What the Celts can't do is allow Middleton or Brogdon or Bledsoe to go off and score a bunch. They need to keep those guys under control. The Celts were fortunate to get the win last night given how effective Middleton was.