The problem with Johnson is he's too predictable...
Rewind to the day we signed him, he came on for City... and he tore everyone apart on the wing, no one knew anything about him or his game. However, he was simply "found out" after that. Every team knows what to expect with someone like Johnson. He's not someone who delivers a ball into the box, he runs at the fullbacks and takes you on... and to do that, he needs space, which is something that isn't granted so easily.
That's the problem with his game and probably the reason why Mancini never played him. Better as an individual than a team player... and I don't mean that mentally, but physically. You never saw him link up in plays with Silva, Toure, Tevez etc. etc.
Hell, even Bellamy when he was with us was whipping them into the box for Tevez when he was here. Johnson's magic was always an individual move and that's pretty much useless unless you're bringing him on as an impact sub... which is what he was most of the time for City.