Vieira came on for the final 30 minutes of City's 2-1 defeat to Hull and although he could not alter the result, the veteran midfielder helped his side look much better after a very poor first half.
With Mancini still concerned by the number of injuries that have hit his squad - paying particular attention to the lack of numbers in midfield - the Italian is looking to Vieira to play a key role in City's push for Champions League football next season.
"I hope Patrick can stay fit for the next 15 games because if we have Patrick at 100% I think he can play very, very hard every game," Mancini said.
"Patrick is an important player with a good mentality. On Saturday he went on the pitch and changed the situation in the middle and I think the whole team can improve with Patrick on the pitch."
Vieira was one of only two signings by Mancini in January - along with young winger Adam Johnson - and Mancini wants him to shore up the centre of midfield where City have had concerns this season.
"I think this is a good squad but we have had more players injured and this is difficult," he said. "In midfield, we have had only three midfielders, Gareth (Barry), Nigel (de Jong) and Stephen (Ireland), and it's impossible that you can play a season with only three midfielders."
Vieira is likely to be involved again tonight against Bolton and the same goes for Johnson, who also made his debut as a substitute at Hull and set up City's goal.
"He's a good player, he played very well," Mancini said. "He's young but can be a good player for us in the future."
Those performances were bright spots in a defeat which saw arguably City's worst first-half performance since Mancini took charge.
The usually cool Italian admitted he had let his players know about it at half-time.
"Yes, I was angry," he said. "This is normal. I was angry after the first half, but I told them if they had a good second half they could recover."
City improved after the break, but could not find an equaliser as Hull lifted themselves out of the drop zone, leaving Mancini in need of three points tomorrow to quickly restore his side's momentum.
"I watch the players every day and when they work they work well and they want to arrive at our targets," he said. "On this I'm very confident.
"I think that Tottenham, Liverpool, Aston Villa, Manchester City and Arsenal are five teams playing for two places and we have a good chance."
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John Gidman's curly perm, Manchester (09/02/2010 at 10:35)
At the moment we have no cover on the right,PETROV is playing out of position and couldn't hit a cows backside with a banjo with his right foot,IRELAND is way below his normal form and BARRY looks like he's either carrying an injury or he is tired and needs a rest.
The signing of VIEIRA and JOHNSON will add much needed variety but we required a creative central midfielder and a box to box player to boost the numbers and give the other players a kick up the backside.
IMO we should have kept WEISS as cover for SWP and added 2 more to the squad.