I actually quite liked him, but others - a couple of whom opinions I value - hate him, so I had a look at some of his more detailed metrics - things like pressures when defending, passes made under pressure etc.
They paint a picture of a player who's metrics have declined somewhat. Now, this could be endemic of a number of things, the amount of football players have been playing in the last 18 months, many of whom having had covid etc. I think he's had an injury and covid (but I might be wrong) I also think playing CM2 systems rarely makes any midfielder look better (unless they are outstandingly multi-faceted - which few are) - and in Milan's chaotic 4231 tactical system this year - where Ibra walks around for 90. minutes and FB's career up the pitch constantly - more so.
And now I'm looking back through his pressure stats (as well as basic stuff like tackles per 90) does suggest he's never been as great without the ball as maybe I thought?
I liked what I saw at Atalanta and previously at Milan - and both sides at played a good brand of football with him intrinsic to their set up - I thought he was a pretty rounded midfielder, not excelling at anything but decent at most things. Technically comfortable, reasonably press resistant, physically robust, good engine - with a reasonable sense of positional discipline and a good mentality - ie composed under pressure.
Maybe he's dipped, or maybe he's just not as good I thought?
Not much help, sorry.