Manchester United legend Ole Gunnar Solskjaer says Manchester City are like a "little brother" who will never take their "big brother's" place.
Ambitious City have spent big in recent years to compete with their rivals, the 19-time English champions.
They pipped their neighbours to last season's Premier League title but United top the table ahead of Sunday's derby at Etihad Stadium.
"When you get disappointments, you bounce back," warned Solskjaer.
Speaking to Football Focus, Solskjaer, who scored 126 goals in 366 appearances in an 11-year career at Old Trafford, said: "It's always this thing about being the big brother and the little brother coming to try to overtake the big brother.
"That always happens in families and in clubs - the young player hoping to take the old player's position - and City are hoping to overtake United. I don't think they'll ever be able to."
Solskjaer, who now manages Norwegian champions Molde, exemplified his former side's will to win when scoring the injury-time goal that captured the 1999 Champions League crown for United in a 2-1 victory over Bayern Munich.
He said United would show that character again to stop City's resurgence, pointing to a solid start in the Premier League. "The start of this season has been good, not brilliant, but United are always improving throughout the season," he said.
"Man United have over the years - and through their history - won in dramatic fashion. In the club's history, you've had so many disappointments, like the [Munich air] disaster, and they have always stood up again and bounced back. It's just in the DNA."
United are three points clear from City at the top of the Premier League table after 15 games, although United manager Sir Alex Ferguson has said he is concerned with his side's recent defensive form.
City remain undefeated in the league so far this season but have been knocked out of European competition following a poor Champions League campaign.