Sir Alex Ferguson taunted Roberto Mancini's U-turn on Carlos Tevez and warned his neighbours he has "plenty of ammunition" if they want to start mind games, as the two Manchester teams battle for the league title, according to a BBC report.
Former City midfielder Patrick Vieira had criticised Ferguson, earlier in the week, saying that United's decision to bring Paul Scholes out of retirement was a desperate act and it showed their "weakness".
However, since Scholes' return, United has won nine of ten league matches, proving the move was a successful one.
"If it's desperation bringing back the best midfielder in Britain for the last 20 years, then I think we can accept that," BBC quoted Ferguson as saying.
"I think he (Vieira) was programmed for that. Roberto had a wee dig a couple of weeks back. We're all going to play our hand that way. There will be plenty of ammunition for that," the Scot added.
Ferguson had the chance to have a go at City when Tevez made his return to competitive football, for the first time since September, after causing controversy by refusing to come on as a substitute in the Champions League match against Bayern Munich.
"If you talk about desperation, they played a player the other night who refused to go on the pitch, the manager said he'd never play again and he takes a five-month holiday in Argentina. What is that? Could that come under the description of desperation?" Ferguson asked.