Man Utd have obvious aim to upgrade squad in January after £5m transfer blunder

Manchester United are closely monitoring the development of Alvaro Carreras after allowing him to join Benfica permanently for just £5million this summer, according to reports. The move proved controversial, and not just because the 21-year-old defender is impressing hugely in Portugal.

United allowed Carreras to end his loan spell at Granada to join Benfica in January, initially on a temporary basis before the permanent deal.

The Spain U21 international's potential was never in doubt as he came through the youth ranks at Carrington, but no first-team opportunities presented themselves.

Erik ten Hag's decision to loan Carreras out as left-back was one of his main problems at Old Trafford has raised eyebrows.

Last season, Tyrell Malacia's pre-season injury and the absence of Luke Shaw prompted United to sign Sergio Reguilon on a short-term loan deal.

But Carreras was there, and rather than recall him from Granada this winter, he was sent to Benfica.

After sending Reguilon back to Tottenham that same month, a recurring injury to Shaw left Ten Hag without a natural left-back for the majority of the dismal campaign.

United did not learn their lesson this summer. They have relied on Shaw and Malacia to once again provide two left-back options, but neither has featured in the first 11 matches as they recover from their respective problems.

Diogo Dalot has covered on the left of Ten Hag's defense this season, with Noussair Mazraoui featuring heavily at right-back following his £17 million move from Bayern Munich.

But Mazraoui's upcoming short-term absence after undergoing a minor procedure and having heart palpitations leaves Ten Hag's options bare.

The Dutchman may not survive the sacking to see out the January transfer window, but whoever is at the helm must consider triggering the buyout clause in Carreras' contract with Benfica.

Although United have struggled in this position, they continue to get stronger.

Carreras has established himself as Bruno Lage's first-choice left-back, appearing in all seven Liga Portugal matches and starting the last five.

He also played a full game in each of Benfica's first two Champions League matches – a 2-1 win over Red Star Belgrade and a 4-0 demolition of Atletico Madrid.

O Jogo reports that Barcelona and Real Madrid are joining United in actively monitoring Carreras' progress. Atletico and Juventus have also been touted as suitors.

But the Red Devils will be the first to secure his signature, and while the amount of the buyout they have agreed with Benfica is unknown, they must make sure they take advantage of the opportunity to fend off the competition.