"You could double what Liverpool paid (£75m) for him," said Lennon.
"It's Liverpool's gain and there must be a lot of people out there kicking themselves about not doing it quicker - because the difference between Liverpool now and what they were defensively before him is night and day.
"He'll just keep going, I think. Who knows where his career will take him? I'm sure he'll want the Champions League and the Premier League, but there's no stopping him, really.
"I agree that he is the best defender in the world. He is. I don't see anyone better than him at the minute. Again, you saw his qualities during the week."