I too think he's been underwhelming this season. However, I think it has more to do with tactics and his position in the team.
We were noticing only Bentancur's good side with Pjanic responsible for all the passing around and playmaking. Last year Sarri got a lot out of Bentancur, and the kid repaid Sarri by being our top assist man in serie A and playing further up the pitch.
This year Pirlo is giving him different responsibilities. He started 9 matches. Once with Artur as his midfield partner, four times with Rabiot, twice with Artur and McKennie, once with McKennie, and once (yesterday) with Rabiot and McKennie.
In 6 of these 9 starts, he played with Rabiot and/or McKennie, and that was a new position and different responsibilities for him compared to those he had with Pjanic in the team. It's not that he was shining when partnered with Artur, far from it actually, but we're talking about 154 minutes of those two playing together in serie A and 82 minutes in CL. That's a total of 236 minutes, or two and a half matches.