Ignoring his defending (which is typically immense but has been more suspect than usual in the past couple of seasons, IMO anyway) - we need his leadership on the pitch now more than ever.
No surprise that the three recent second-half capitulations have been when he wasn't playing against Bournemouth, Sporting and Brighton. When the going gets tough then we need somebody on there to act like a captain, even if he doesn't have the armband, and rally the players.
We don't have that at the moment. The sooner he's back, the better.