Aguero hasn't played a long time and hasn't scored for 13 months a league goal. He's rusty. Even Kevin was very rusty against West Ham. Fernandinho at 35 is not the same player. Of course, such things matter. We were very close to dropping points vs West Ham precisely because those players weren't in top condition.
See the team that played Wigan in 2018. Wasn't bad at all, but lost.
Edit: That's the team: Bravo, Danilo, Stones, Laporte, Delph, Fernandinho, Gundogan, Silva, Bernardo, Aguero, Sane. Absolute quality on paper.