Aguero was missing from the City team that drew at Stoke on Saturday with a foot problem.
Mancini has refused to say exactly what happened. However, it seems the problem was not of an orthodox nature.
"It is a stupid injury," said Mancini. "It was not his fault but he can't play with this injury; for one day, 10 days or two weeks. I don't know."
Mancini was repeatedly asked what Aguero has done, but declined to elaborate each time.
"I prefer not to say," he said. "But he couldn't play against Stoke and can't play tomorrow. I hope he can recover for Arsenal next weekend."