Pace is one of the most important attributes in a modern full/wing back. Lose that, you lose the ability to get back behind the ball, to get into a good position to close down and tackle as well as blunting your attacking ability. That’s without highlighting the less obvious physical health, mental health, motivational and confidence issues that serious and/or lengthy injuries can have on players. They’re not machines.
Doesn’t matter how many coaches he’s worked under since Wenger, if he’s lost an essential ingredient because of injuries, he’s never getting back to where he was. See Emile Smith-Rowe for details.
I think you might be discounting how much easier it would have been to develop those extra skills while still having insane pace.
I’m not giving him a total pass for failing to do it, but if he’s gone from having potentially elite pace (which I think is debatable itself) to having just pretty good pace, your opposing fullback or winger don’t really have to stress as much so they can be ready for something other than a push and run.