Paul Merson says Manchester United would become serious title contenders if they beat Manchester City on Sunday.
Premier League leaders City hold a two-point lead over United and have won four of the last five derby clashes.
However, Merson was impressed with the way United bounced back from their 3-0 defeat at Arsenal with victory by the same scoreline against Everton last weekend.
"A win in the derby would be massive for United," Merson told Sky Sports News HQ. "They have a hard month and they started off with a great result at Everton.
"If they can beat Man City they are massive players in the title race. I don't see Man United losing this game."
City have not kept a clean sheet since September 12, and there is debate over who will start in the centre of defence, as club captain Vincent Kompany has been on the bench for the last two matches with Manuel Pellegrini preferring Eliaquim Mangala and Nicolas Otamendi.
But former Arsenal midfielder Merson says Kompany should return to improve City's defence if they are going to come out on top against United.
"I watched them [Man City] against Sevilla [in the Champions League] and they turn up like they are playing Bournemouth every week - no disrespect to Bournemouth," Merson added. "They just open the game up and if they do that against Man United then they could be picked off.
"They have to be a bit more tactically cute and not go in and say in 'you have a shot, we have a shot'.
"If you're a Man United fan you would be more than happy if Kompany doesn't play because the two centre-halves on Wednesday were all over the place. They are going to give Man United chances."