Nicolas Jackson took advantage of errors from the pair as Villa won 2-1 on Saturday, ending City's perfect run of seven wins from seven league games.
Defeat left City level on points with champions Arsenal at the top of the table, but they are ahead of the Gunners on goal difference.
Maresca spoke up for his players ahead of a tough League Cup fourth-round match against Brighton at the Etihad on Wednesday.
"Gigio (Donnarumma), I think, since we started, is doing well. Like any player, like any manager, at some point is making some mistakes.
"But it's part of the season. Again, they are playing so many games, so many minutes, that sometimes it's almost impossible to (not) make some mistakes."
And Maresca said he was "completely happy" with England defender Guehi.
"Because in the way he reacts, he's fantastic, he's a top guy," he said. "Again, he's one of the captains, he's a leader, and I love Marc, to be honest.
"And again, it's the same with Gigio - when they make some mistakes, they recognise."
Maresca said Donnarumma was among a group of players who had been given time off to recharge after the Villa defeat, also including Erling Haaland, Elliot Anderson, Josko Gvardiol and captain Ruben Dias.
"After Villa, we gave five players three days off to recover energy -- Erling, Ruben, Elliot, Gigio and Josko," he said. "They are all on holiday."
