"People maybe don't realise the situation and how well he's playing," said the Reds boss.
"I could tell you he's doing really well and leave it at that, but you also have to remember that since he's been here the games have been United, Everton and Man City away. That makes how he's doing even more impressive.
"Spain has a very good group of players, so it's not easy - but he can be there. If he keeps playing well here, it will be easy for him.
"I think playing in the Premier League and Champions League can also make a massive difference to the progression of a player."