"He's been fantastic for Manchester United," said the Liverpool boss. "I think he's even surpassed the legend that was Sir Matt Busby in the achievements and successes he's had.
"To be anywhere for 25 years is unbelievable, and to have had the success he's had is great testimony to himself and the football club.
"When you remember where the football club was when he came in, it's a fantastic achievement where he's taken them to.
"I don't think you'd have seen Fergie lasting 25 years until Mark Robins scored a header at Notts Forest, so who knows?
"Whether anyone does it in the future is irrelevant - nobody can take away his achievement."