Former Liverpool great, Mark Lawrenson questions Sir Alex Ferguson's failure to replace Cristiano Ronaldo.

In his column for Daily Mirror, Lawrenson suggested that the problem with Manchester United this season is that they are missing Ronaldo with no proper replacement brought in to ensure a smooth transition.

He said: "Sir Alex Ferguson always knew the year after selling the world's best player would be a tricky one.

"But in previous campaigns the difference would have been Cristiano Ronaldo would have been replaced by a marquee signing.

"Ronaldo could score 40 goals in a season, went for £80million and yet was replaced by a player from Wigan.

"As well as Antonio Valencia has done this season, he is not Ronaldo. He is not a direct player who runs at defences and can single-handedly destroy teams in the way Ronaldo did."

Lawrenson was also quick to point out how the both the senior players and youngsters have let Ferguson down in this campaign.

He added: "This season, United have lacked that same cut and thrust. Valencia has done well but is not the same. Ryan Giggs often comes on but doesn't run at teams like he used to.

"Dimitar Berbatov is very patchy and inconsistent. If you tie Wayne Rooney up in knots then United are not nearly as effective.

"They have lost that spark, that magic, that extra ten per cent which would really see off opponents every other week.

"Fergie has given the young and fringe players a chance and they have not come up to scratch. You can bet that's hurt him the most. Some of them did not look hungry enough.

"Ferguson, however, is a proud man. There can be no question of him retiring at the end of the season. He will have to be carried out of Old Trafford in a box. He loves the club too much.

"Fergie also knows that underneath there is a group of emerging players. The Brazilian full backs, Rafael and Fabio, Rooney has his best years in front of him. Jonny Evans and Darron Gibson are going to be good players.

"But at this level, the missing ten per cent from the team being weakened by Ronaldo's sale makes all of the difference."

Fergie's hopes of retaining the league title suffered another dent on Saturday night when Birmingham held the Red Devils to a 1-1 draw at St Andrews.