I don't think Pervis had the best season, but he was fine enough (certainly not a liability), played the 4th most minutes out of our outfield players, and covered a hell of a lot of distance.
FWIW, this is how he compares to other fullbacks in the top leagues for a few key stats.
Attacking output isn't just goals and assists. For fullbacks, that is usually going to be pretty low.
The way he helps us attack down the left hand side with his ball carrying, take-ons, forward passing and overlapping runs is key to our attacking success.