I've noticed that there are a lot of knee-jerk reactions to our little blip several days ago. Some of the are unfounded, but I quite agree that we really could use another winger. However, we aren't crying out for one. We're beating teams left and right and really do have a competent squad.
I do feel though a new winger (a very good one; not necessarily world class), even a young one with bags of potential would give us another dimension that we will really aid us in the future.
There are some times in football, even for traditionally posession-heavy build up teams like Arsenal and Barcelona, sometimes it really helps to have a bit of directness. A pacy, skillfull, and direct winger would get past a fullback with the ball and contributing in the final third with: crosses, passes in the penalty area, and shots on goal.
Take a look at teams like United and Madrid. Although their playing style is quite different from ours (they move the ball quickly through the wings primarily), when teams sit back and defend against them, they fall into a trap. Why? Because if you already don't have the ball and you're sitting back, if a winger can get in behind the defensive line multiple times, then chances are if you have quality on your team, you can capitalise on this and score a goal.
I think Mancini realises the need for an out and out winger that can consistently beat a man is evidenced by his dogged pursuit of Alexis Sanchez which proved to be futile.
We have our way of playing, yes, but we also need a Plan B; a player that can change things dramatically. Specifically, a real winger, not just another quality pair of fresh legs to come in the game because we have plenty of that.
Johnno is supposed to be that spark, but is really hot and cold. Remember last year? Mancini said: "After Adam Johnson got injured we had a big problem because he is different to the other players. We don't have anyone else like that but he won't be back for another month."
I think Bobby knows we could use a winger. Contrary to the OP, I think we only need one though.
There are plenty of quality wingers to pick from:
Experienced: Lavezzi, etc
Young: Hazard, Hoillet, Reus, etc
I just hope that when Tevez is sold, we buy someone.