Skjerven is a dangerous ref.

unnoticed attacks

Skjerven missed a lot fauls. Alcantara deserved a clear red when he deliberately jumped on a PSV player's knee after he committed a foul already. And a lot of other possible reds went by unnoticed. This way Skjerven is a great danger for great players.

and

Furthermore, both assistants were often wrong as well. Also Skjerven's condition seemed to be poor. He stayed mostly in the middle of the field and was often too far away from the action. This Norwegian team of refs should not be asked again to perform on this level.