On his first appearance after two months out with a calf problem, Kompany netted City's third in a 4-0 triumph over Newcastle that consolidates their second-placed position in the table.
Little wonder Mancini was so concerned at Kompany being called into World Cup action for Belgium last week, given the captain's obvious importance to the Blues.
"Vinnie is important for us," said Mancini. "We played without him for 60 days, which was very bad for us, like when we lost Yaya Toure for a month. "There are some players that are very important and if they don't play for three, four, five games it is a big problem."
Kompany turned home a Gareth Barry strike at the start of the second half to snuff out any hope of a Newcastle revival after Carlos Tevez - with his seventh in six games - and David Silva had put the hosts in command before half-time.
Yaya Toure increased City's lead before the end and with Sergio Aguero also returning from a knee injury, the Blues will head into their April 8 Manchester derby meeting with United at Old Trafford in confident mood, even if their Premier League title defence is almost over.
"It is impossible we can win the title but it is important to finish the season well," said Mancini, whose side will discover their FA Cup semi-final opponents on Monday when Chelsea entertain United.
"If we play like this we won't have a problem in the last couple of months of the season.
"We want that second place and to win the FA Cup semi-final."
Mancini confirmed defender Matija Nastastic had been ruled out by a foot injury sustained on international duty, although the Serbian is not expected to be missing for long.
Newcastle manager Alan Pardew admitted his side were second best, a situation that would have been the same even if they had been at full strength.
"I think we might have struggled against that Manchester City side even with my best team," he said. "Kompany gives them an assurance and a presence at the back that they might have been missing in other games.
"I don't think City are going to win the league and in the first half they had an assurance about them that comes from playing with no pressure."
The same cannot be said of Newcastle. Whilst Thursday's Europa League quarter-final with Benfica sprinkles some stardust on their season, the spectre of relegation is growing.
The Magpies are now just three points above the drop zone and Pardew accepts they have some scrapping ahead of them if his side are to preserve their top-flight status.
"I don't think we ever thought we were out of it," said Pardew.