Manchester United may have made a catastrophic error in the closing stages of the transfer window...

What's the word?

According to Fabrizio Romano, the Red Devils have reached an agreement with AS Roma to sell Chris Smalling.

The Italian transfer expert insists the Roma board had been working on a deal for the past three months.

And the paperwork was signed with a few minutes left to spare.

Romano revealed the transfer will set the Serie A side back €15m, which equates to £13m, with add ons as well.

Victor Lindelof struggled against Crystal Palace so Ole Gunnar Solskjaer rang several changes, and after a humiliating defeat to Tottenham Hotspur on Sunday evening, the United boss could be forgiven for feeling a little let down by his team's abysmal performance.

Could prove to be costly

The former Benfica centre-back has been a regular feature alongside club captain Harry Maguire in the middle of the defence but whether he is actually good enough to play on a permanent basis remains to be seen.

Despite dropping down the pecking order after the arrival of Maguire, Smalling's untimely departure could prove to be costly.

Both Eric Bailly and Maguire suffered from a defensive meltdown as Spurs hit back in style to crush Solskjaer's side, capitalising on several high-profile mistakes from the hosts to score six goals and ensure Jose Mourinho enjoyed a triumphant return to Old Trafford.

Maguire has never been particularly reliable at the best of times but his errors in the build-up to Tottenham's equaliser established the tone for the rest of his afternoon.

To make matters worse, the 26-cap England international committed the foul which led to Tottenham's second goal only three minutes later, and he never really recovered.

And it's not like Smalling would have been a bad option on the bench either, or even the starting lineup, as Paulo Fonseca has been singing his praises.

Last season, the 30-year-old made 30 appearances in Serie A, scoring 3 goals and supplying 2 assists, whilst he made 4 clearances and 1.9 interceptions per game, via WhoScored.

By comparison, Lindelof could only manage 3.5 clearances and less than one interception per game, as per WhoScored.

Sir Alex Ferguson himself once described the defender as "fantastic" as the legendary manager insisted he was the foundation of the club.

How the times have changed...

When we look back at it all, will selling Smalling have really been worth it in the long run? Let's face it, no, probably not.

The Red Devils may regret making this unforgivable transfer blunder.

AND in other news, Manchester United must sign in-demand Watford winger Ismaila Sarr by any means necessary...