Alan Shearer has sent a stark warning to Manchester United boss Ruben Amorim ahead of this weekend’s clash with Chelsea at Old Trafford.

Writing in his column previewing the Premier League fixtures, Shearer didn’t hold back in his criticism of United’s performance in last weekend’s derby against Manchester City.

He wrote: “It was men against boys last weekend at the Etihad in the Manchester derby. The teams were miles apart in terms of ability, system and attitude, despite Manchester United spending over £200m in the summer.”

Shearer made it clear that Amorim’s side cannot afford a repeat when they face Enzo Maresca’s Chelsea.

He went on: “They cannot afford to be beaten like that again this weekend when they host Chelsea. If United lose to Enzo Maresca’s Blues then I fear for [Ruben] Amorim.”

The former Newcastle striker believes something has to change quickly at Old Trafford.

“Something must improve rapidly. United can’t be in the bottom-half of the Premier League. There hasn’t been an improvement at all up to now and that can’t continue after what they spent in the transfer window.”

Shearer questioned United’s lack of progress despite heavy investment and stressed that results must turn around immediately: “There has to be something that gets them going and if that doesn’t happen quickly then everyone knows the rules when it comes to football management.”

According to Shearer, the responsibility lies with both Amorim and his players, but he pointed to the manager’s system as a key issue.

He wrote: “It’s on both the manager and players but I do think there’s a lot in the system and the manager is not going to change his ways. He’s either going to die on his sword or get the success that he needs.”

At present, Shearer believes Amorim’s tactical approach is simply not working for United, as he concluded: “At the minute it looks really negative. I don’t think a lot of the United players are suited to the system.”