He's been average for a couple of years now. Ok, it's late Wenger Arsenal and you can understand why he might be demotivated but United are not exactly Real Madrid level yet. As for what he does for Germany, international football is generally of a poor standard. It's definitely an upgrade on current no.10 options but, if we can get Griezmann next June, I'd take the opportunity cost for the second half of this season in favour of a younger, hungrier player next summer.
That's a bit of exaggeration.
In 2015/2016 he recorded 19 assists in the league alone, with 6 goals scored as well.
His form dropped last season only.