The 22-year-old midfielder contributed to a promising City peformance in the first half, when City battled for possession and used patient build-up play.
But Keegan felt Barton was off the pace in the second half, when he gave the ball away a number of times due to misjudged passes, leading to his 68th-minute substitution for Jon Macken.
However, the City boss was quick to point out that Barton is still a developing talent, and that he has confidence in the youngster getting back to his best.
"Joey has done very well for us but he is not the finished article," Keegan told the official City website.
"Joey is not everything that he wants to be or that we want him to be yet.
"He had a spell in the game in the second half, like Willo Flood did a few weeks ago, where his inexperience was starting to hurt us as well as himself. So I took him off but I am in no doubt that he will bounce back from it."
Barton had established a solid centre-midfield partnership with Paul Bosvelt, and the most likely explanation for his below-par display on Sunday was the absence of the veteran Dutchman by his side, although you could argue that Barton wilted under the derby pressure.