Millsap is a 4 who can do everything. He's an excellent fit and fallback if Hayward isn't signed.
But Denver is going to push to get him signed ASAP.
One thing Ainge is poor at is TIMING. He gets beaten to the punch by other teams. He should be into Millsap now as a backup to Hayward. Sure, Millsap is 32, but he's in good shape, a real pro, and a 3 year deal would be fine. He works with Horford, and would be a nice mentor for Tatum and Brown.
I'd rather see Millsap than Gallinari, who is a walking wounded with bad knees, and can't play defense.