With City dominating the early stages but without creating many decent chances, Shaun Wright-Phillips finally managed to fashion a clear opportunity in the 36th-minute after beating Wes Brown on the right side of the penalty area to put a low cross into the danger zone.
But McManaman, making his first start for three months in place of the suspended Paul Bosvelt, somehow succeeded in sidefooting the ball wide when unmarked, three yards in front of the near-post.
The City boss praised the 33-year-old midfielder - still to score in a City shirt - for getting into the position, but acknowledged the significance of his miss.
ββHe was there to miss it,β is what you have to say in situations like this," Keegan told the official City website.
"I know he is disappointed. It was a chance that you would normally expect him to score from.
"If it had gone in it would have changed the rest of the derby - I am sure of that."