I can see why you and others weren't impressed with his performances against Celta and Anderlecht but I thought he did fine. Just personal preference, I guess.
I'll never argue that he loses the ball too easily because he does, and it was one of the main reasons I wasn't keen on signing him. I agree, we need our best players to step up in these matches but our tactics, IMO, prevent them from doing so. Whether it was Rashford, Martial or Mkhi up front against City they were completely isolated and ended up conceding possession due to our whole team being camped. Heck, we employed the same tactics away to Liverpool and Zlatan struggled, too.