The Uruguayan continued his sensational 2013-14 season with a hat-trick in the Reds' 6-3 win in Wales on Saturday, taking his tally to 28 goals in 25 appearances.
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"There's Messi and then Ronaldo - that is it for the best, best players [in the world] because they have just hit a level that I don't think any of us have ever seen; these guys are hitting 40 or 50 goals a season regularly," he told Liverpool's official website.
"Then there's a group just beneath that and Luis is at the top of that group. There's no two ways about that.
"I had the opportunity and the pleasure to train with him and play with him and he was outstanding then. The better the team has got, it has just allowed Luis to flourish as well.
"Liverpool have a lot of top players – [Daniel] Sturridge is brilliant and there's Stevie [Steven Gerrard], who is just immense - but, with a player like Luis there, you have an opportunity to win every single game you go into because he is that good."