I've seen him lots for Milan and in particular against the Irish U21s. None of what I've seen make him look like a great player. His 'name' is apparently worth much more than what his current level is. Maybe Howe can turn potential into reality but they have likely paid 60m+ for him and that makes the Mount deal look like an absolute bargain. Mount has shown a much higher level of performance so far.
How much is "lots"? Milan fans on Reddit aren't happy about his departure at all, and with all respect, I'd value their opinion more.
Howe's system made Longstaff and Willock look like top midfielders for large parts of last season (by the "eye test" at least), consistently bossing and overrunning supposedly better players and better midfields. From what I gather, Tonali is a massive upgrade on Longstaff, bringing the same engine but with some actual quality. If that is the case, I can't see this being a bad singing.