"In my opinion, our spine is as good as anything in the world. If you look at the quality we have down the spine, it is fantastic. We have other good players as well, though," said the Reds captain.
"It's difficult to say how many world-class players we need to buy. You look at the points difference and over the last few years we have made it smaller.
"Playing against the Arsenals and Chelsea, it has been close. We are giving them good games which are tight and get decided on small details.
"If we can make the team a little bit stronger, have a few more options and match-winners, and make the team a tiny bit better defensively, we can do it.
"Other teams are going to strengthen and become better sides, so it is important we do the same.
"There is a lot of money in the league - teams and squads will become stronger in the summer. It's important we do not fall away, we want to stay up there with the best.
"To keep doing well in the league and Europe we need to strengthen the squad, there is no doubt about that."