Maybe Monk saw that the ref was giving lots of fouls (some correct, some not) and decided it wasn't the correct 'environment' for Fernando with how the ref was.
I mean, Fernando has something to prove to get back into the team, we know he can be persistent and a nuisance to the opposition player, hounding them down.
He possibly thought with the ref giving 'niggly' fouls he would end up booked or worse.