Regressed is too strong a word, like the other players he's just not performing to the best of his ability that can quickly change with some confidence. Agree with Don too, Tierney is probably the one player who's actually benefitted from Arteta's system in the past.
Unlike others though my concern for Tierney is that it seems to be a bit of a pattern for him. He seems to be another one who's following the Ramsey cycle of spending 1/3 of the season looking great, 1/3 lacking sharpness and another 1/3 injured. There's stories of him approaching training like it's match day and while it's great to read, maybe it's taking it's toll on his body.