In theory he has the ability to be as useful attacking wise to us as Alexander-Arnold was last season for Liverpool. Mendy is a great attacking talent, in theory. People fixate on his crossing, but he has this wonderful ability to play the ball inside at pace as a pass to a central player, such as for the Aguero goal. He's also a surprisingly decent player at beating fullbacks with skill in one on ones.Not sure about the lesser games, he has the ability to create chances over & over again.
And regarding his injury, I am not sure if his record is worse than Gundogan's record.
His injury record has been better this season, certainly.
I think Mendy has an extremely high ceiling (which as I've stated elsewhere, is the reason I think he's still not been loaned out anywhere or ever been put up for sale). The difficult part is him getting there.