The England striker has questioned United's ambition after confirming his desire to quit the club and indicating he will not sign a new contract.
"Players have opinions and if they want to leave then good luck to them," the Ipswich boss and former United captain said at a press conference shown on Sky Sports News.
"Man Utd will more than survive."
Asked if Sir Alex Ferguson's side can still attract top quality players, the Irishman added: "Absolutely, of course, absolutely.
"If any player ever asked me, and one or two have, if they had a chance of signing for Man United, 'do it, you won't regret it'.
"I was there for 12 and a half years I loved every minute of every day playing for that club, every minute of every day
"I had chances to leave, many opportunities when my contract situation was running out, particularly when I came back from my cruciate (ligament injury).
"An amazing football club, brilliant football club."