Although Carlos Tevez and Emmanuel Adebayor scored to leave City trailing fourth-placed Liverpool on goal difference, £8million January transfer window signing Johnson claimed the man-of-the-match honours.
The former Middlesbrough man unhinged the Bolton defence with his direct running on a night when the rest of the Blues seemed strangely below par.
Added to that ability, Johnson can also pick out a decent cross, which makes him a rare breed indeed in the modern game.
"That is why we bought him," declared Mancini.
"He is a young player who wants to do well. He can play on both the left and the right and he can also play behind the main striker.
"He can continue to grow and I think he can be a very important player for this club."
Johnson claimed the assist for Tevez's 19th goal of the season when he was brought down by Paul Robinson on the half hour.
Bolton boss Owen Coyle claimed it was a "soft" penalty. The tackle actually looked ridiculous given Robinson made no contact as he lunged in on an opponent who appeared to be heading over the by-line.
Adebayor sealed victory with a brilliant finish to a pass from another new boy in Patrick Vieira.
At 33, it is felt by many Vieira's best days are behind him.
Yet the Frenchman bristles at such suggestions and reacted with a curt "am I that old?" when he was asked after the game whether he would need a rest on Wednesday.
Mancini can see Vieira's point and thinks the midfielder can follow his own example and play until he is 36.
"I played until I was 36 and I was still playing well," countered the Italian.
"I won the Scudetto at that age, so I see no reason why Patrick can't carry on as well.
"First of all I hope he can play another 15 games for us in the league this season because his performances are going to be very important."
Vieira is also aware he has a big part to play but is determined to send City into the Champions League next season.
"It was my first 90 minutes for quite a long time and we won the game, which was really important after losing at Hull on Saturday.
"There is a long way to go and we need to keep winning games. Our target is the top four and we believe we are strong enough to get there."
Mancini confirmed Craig Bellamy should be fit for next Tuesday's visit to Stoke after having his knee assessed by a specialist in London on Monday.
The City boss also played down fears over skipper Kolo Toure, who limped off with a knee injury at the start of the second-half.
Bolton remain precariously placed, just a point above the drop-zone, although Coyle can see grounds for optimism.
"City have a squad that cost over £200million and we made them look ordinary," he argued.
"If they are honest, they will admit they were fortunate."