City missed out on the title to Arsenal last season and are expected to be the closest challengers again.
Maresca’s men remain top on goal difference but will be overtaken by the Gunners if they avoid defeat at Liverpool on Sunday (kick-off 16:30 GMT).
City have conceded 10 goals in nine league games this term – double the amount that Mikel Arteta’s men have let in.
Individual errors at the back are beginning to stack up – Marc Guehi, who was dropped for this game, and Gianluigi Donnarumma were culpable at Villa Park and Matheus Nunes was the guilty party against Brighton.
Maresca has been unequivocal about his number one, saying it is “Donnarumma plus 10” on the teamsheet, but defensive frailties mean City have now kept only one clean sheet in their past eight games in all competitions.
The result also meant City lost consecutive Premier League games for the first time in 14 months.
“Why?” Maresca asked of the individual errors. “Sometimes it happens.
“Unfortunately it is happening for our players, but I think it is part of the game, part of football. It happens many times against us.
“We need to analyse what we can do better and try to move on.”