The Irishman, who picked up 11 red cards during his career at Old Trafford, said that he believes players are now "being nice to one another" instead of dishing out hard tackles.
"There's a skill in tackling and nailing people," Keane said in an interview with the BBC. "Yellow cards are fine, you need to worry about the red ones. You can still leave your mark on a player but when I see United games now, even against Arsenal, everyone's being nice to one another. Everyone's gone a bit soft."
Despite his assessment, the former Sunderland and Ipswich Town manager said that he expects Sunday's match at Old Trafford to be fiercely contested.
"I don't think it will be a cagey affair," he added.
"United will be hurting from the semi-final (last season's FA Cup defeat) and they will want to keep City in their place. If City won on Sunday that would shock everyone and that's why they need to be kept in their place.
"I think the progress City have made, if I was still playing for United, City would be the game I would want to win more than anything else. They're serious challengers for the league."
Keane was sent off in a derby for United in 2001 for his knee-high tackle on Alf-Inge Haaland.