The Spaniard has 10 goals in 16 games following his brace against Porto on Wednesday night.

"He is a world-class striker and he's probably up there with anyone now in striking terms. I only have to mark him in training, thankfully," the 29-year-old told Liverpoolfc.tv.

"It's raw pace and strength that he's got and that will cause defences problems.

"It was a big relief when he got the second goal because when it's 1-1 anything can happen. It was a great goal by Fernando, showing what a top player he is. It was a great performance from him. He's been outstanding all season but he's done it in the Champions League and in a big pressure game like that.

"He looks like becoming the 20-goal a season striker we've been crying out for but it's not just that, it's the work he puts in. He puts himself about and I think people appreciate the graft he puts in. Even if he doesn't score he makes it really tough for defenders.

"He's a great lad as well but as long as he's scoring goals it doesn't matter what he is!"