From £1.8m to £40m: SAFC endured howler over gem who was "the best in Europe" - opinion

Sunderland are no stranger to a transfer mistake, as they have made a number of blunders over the years in the market.

Some of them were out there for all to see as the Netflix documentary illustrated their deal for Will Grigg – which flopped badly – in all of its glory.

However, they have all had howlers in the sense that they have not been able to land a player who has subsequently gone on to shine elsewhere.

One such player was left-back Marcos Alonso in 2014. He spent the second half of the 2013/14 campaign on loan at the club from Fiorentina and enjoyed his spell on Wearside, playing 20 matches, including a League Cup final defeat to Manchester City.

Speaking about his time at Sunderland, Alonso said in 2020: “I had six very happy months at Sunderland.

“When I went there we were bottom of the table but we ended up safe from relegation and there was also the League Cup final. That was special even if we could not win. That day at Wembley will always be good memories for me.

“The fans they enjoyed that day so much. I think even more so than we did. We lost in the end, but their support was amazing – much better than the City fans – and for me, that was very special.”

Despite the success of the loan move, the Black Cats were unable to strike a permanent deal for the defender, who was valued at £1.8m by Transfermarkt at the time.

He stayed with the Italian outfit for two more seasons before sealing a transfer to Chelsea in 2016, and that is where his career took off.

The Spaniard caught the eye in his first two campaigns at Stamford Bridge as he racked up 13 goals and five assists in two Premier League seasons for the Blues, winning the title in 2016/17 under Antonio Conte.

In 2018, then-Chelsea boss Maurizio Sarri