Donnarumma, who arrived from Paris Saint-Germain on deadline day, was handed his first start in Sunday's derby and responded with a commanding display. The 26-year-old kept a clean sheet and produced a crucial save in the second half when United threatened a fightback.
"Don't forget Gianluigi Donnarumma at the back, it's easy to not speak about him because all the others played so well," Haaland said. "But he had an amazing game and a clean sheet. A fantastic debut for him. He's one of the most experienced players in the squad even though he is only 26.
"He started playing when he was 16. He's an amazing keeper and he's going to be massively important for us."
Haaland, who scored twice and was named Player of the Match, also reserved praise for Jeremy Doku. The Belgian winger was a constant threat, setting up Phil Foden for the opener before creating Haaland's first goal and supplying another chance just before the Norwegian added his second.
"Jeremy played really well, it's amazing to see this," Haaland said. "This is his level, and this is what we know he can do. He has to continue at this level."
The victory offered a welcome lift for City after a stuttering start to the Premier League season, with defeats against Tottenham and Brighton leaving Pep Guardiola's side with two wins and two losses from their opening four matches.
Guardiola has urged his players to build on the momentum, with a Champions League clash against Napoli awaiting on Thursday night at the Etihad.
"We know how important it is to find consistency," Guardiola has said. "The derby was a big step forward, but Napoli will be another very difficult challenge."