City delighted to have Kompany back
The Belgian international returns after being given a four-match ban for a clumsy two-footed challenge on Manchester United winger Nani at the start of the month.
"Vinny is an important defender for us, a leader," Mancini told reporters on Monday.
"He is a strong defender and has improved a lot in the last two to three years. He is one of the best in Europe."
City go into the game holding a three-point lead over second-placed United, who entertain Stoke City at Old Trafford.
Mancini said it was now a straight fight between both Manchester teams after City defeated third-placed Tottenham Hotspur 3-2 on January 22, the previous round of league fixtures before last weekend's FA Cup ties.
Asked if it was now a two-horse race, the Italian said: "Yes, I think so. I think it will be very difficult for the others because maybe it's impossible that us and United lose all our remaining matches.
"Of course over 16 games anything can happen but at the moment it's between City and United."
Kompany believes his side are capable of winning all their remaining fixtures despite being depleted in attack with striker Mario Balotelli still being banned and the Carlos Tevez saga dragging on.
"There is a belief in the City team we can become even stronger towards the end of the season," said the defender.
"We are set up quite nicely for the finish. I don't see why we cannot win all our remaining games.
"If we just keep doing what we're doing and don't get sidetracked with all the talk in the newspapers and such like, teams will find it hard to beat us," added Kompany.
United manager Sir Alex Ferguson is looking forward to the match against Stoke, a team he said had the same sort of never-say-die fighting spirit as 1988 FA Cup winners Wimbledon.
"You know what Tony Pulis's teams are like, they are always the same with 100 percent commitment," said Ferguson. "They all get stuck in and you have to cope with it - I quite enjoy that.
"When we used to play Wimbledon... when they first came into the top division... nobody looked forward to playing them. But then my team started to enjoy it.
"It was a physical challenge in the way they played and a physical challenge in terms of trying to take control of the possession and play your football," added Ferguson.
Spurs, five points adrift of United, host bottom club Wigan Athletic at White Hart Lane while fourth-placed Chelsea are expected to give a debut to England defender Gary Cahill at Swansea City in the absence of injured skipper John Terry.
Terry, who has fluid on the knee, is ruled out along with midfielders Ramires and Frank Lampard.
"It's just a bone oedema that he has been carrying for some time since he collided with a post, if I am not wrong against Portsmouth, and this stops him a little bit," manager Andre Villas-Boas told Chelsea's website.