If you judge him on his bad passes then yes he's been very inconsistent since he's come back but if you're actually paying attention, he's made a lot of good passes too. His first time balls freed up Antony and Bruno quite a few times, which could have had been assists or pre-assists. Also it was his cross for Martial's header, which he should have done much better with; on a separate note, Casemiro's crossing has been very clutch this season as he's delivered quite a few clear cut chances, which should have ended with goals.
For the way we play, which is very transitional and about exploiting the space after we've counter pressed etc, Casemiro is playing his part. He's very high risk and for sure we could do with more consistency on the passing front (in terms of his choices) but for the most part he's been decent.
Until we get another proper midfielder, we'll have take the (plenty of) good with the bad.