Some expected City to challenge for a Champions League spot after a fantastic start to the season but they dropped into the bottom half for the first time this term after last week's derby defeat.
"The manager has taken this club from the middle of the First Division into one that is expected to get into Europe, and that's all credit to him (Keegan)," said Barton.
"People have been saying that he might do this, he might do that, but every player is 110% behind the manager because we believe in our own ability and we believe it is only a matter of time before things start to go our way then everyone will forget about what has been said in the last few weeks.
"People are nit-picking because results are not going our way but we have got to be bigger and better than that.
The City midfielder, one of the side's few plus points in recent weeks, then dismissed talk of tension amongst the playing staff.
He said: "We have a lot more things to worry about than talk of a rift between the French and whoever. It is frustrating but it comes with the territory and, like I said, we have got to be better than that and prove the doubters wrong.
"As far as the players are concerned it is business as usual here and we are looking up the table not down it."
"Teams who are going to play us now are may be going to think that they are going to take points off us because we are low on confidence but I think United found out in the second half of the derby that we are not going to lie down."