Diggs is actually not good on separation. According to nextgenstats, Diggs avg cusion on catches is 2.7 yards. Not only is that not very good compared to other receivers, but it's not even good compared to our own. It's pretty much the worst on our team. Even Davis is 3.0 avg. That said, Diggs is still elite IMO. Still our best WR. Still a top WR in the league. He has just made some mistakes these past few games. All receivers make mistakes though.
There is also no way Diggs is "checked out." The dude is still getting fired up and being a leader on the sidelines and stuff. Thats not checked out.