United have failed to replace the Irishman since he left the club almost ten years ago.
Rooney lauded his former team-mate, saying: "I knew he was a great footballer, but until I played with him didn't realise how great.
"His passing was probably the best I've played with. In terms of getting it in to the forwards' feet.
"I had associated him with someone who got round the pitch, making tackles, but it was his passing that blew me away."
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