Keegan rates the big Geordie ahead of his former Reds teammate John Toshack.
"What people tend to forget about Andy is that he's still only young," Keegan told liverpoolfc.tv.
"I played alongside Tosh when he was at his peak but he was in his mid-to-late twenties at that point. Therefore Andy's best days are ahead of him and there's still a way to go before he fully reaches his potential. What I would say though, and this is meant as no disrespect to Tosh who was an excellent clever player, is that Andy is better in the air than he was. He's outstanding, probably the best I've ever seen."