We need better than Otamendi at CB to win the league. The way he was bullied by Costa for Chelsea's first two goals was nowhere near good enough. Chelsea's first goal simply wasn't good enough from Otamendi, a proper centre back deals with that first ball comfortably. He's miles away from Van Dijk's level, who we need to sign in January.
We aren't scoring enough goals because we haven't got enough goalscorers in the team, it's as simple as that. Only Aguero was a natural goalscorer today IMO. Silva, Gundogan, Fernandinho & Navas - none of them are natural goalscorers. When they're all in the team, we'll pass the ball around really well and create good chances, but rely extremely heavily on Aguero to finish them. We have players in the squad who are great finishers though, the likes of Iheanacho, Yaya, Nolito and Sterling.