"There is no doubting his quality but we have to look at what his performances will be next season," said the Liverpool boss.
"Maybe you can find fantastic players of, say, 23 years old who also have fantastic pace and quality.
"Robbie is top class, no doubt about it, but you also need pace and stamina levels if you want to win trophies.
"We are really happy with him because he is working so hard. But we have to know exactly how many top-class players we need and if he could be the top-class one we are looking for.
"It's not that we don't believe in him but this is a big club and we need to look at different players as well.
"It's really important to have players of the passion he has so it is all in the balance.
"We need to put all those things together and weigh them up before we decide.
"Robbie knows he needs to improve some things because he wasn't playing for some time and this is another level again."