Chelsea were perhaps always going to rue letting Antonio Rudiger leave on a free transfer last summer but the German's impressive form ahead of the Champions League quarter-final next month will hammer home the Blues' mistake.

How much is Antonio Rudiger worth now?

Chelsea signed the powerful centre-back from Roma in the summer of 2017 in a deal worth £29m, with the German defender signing a five-year contract at Stamford Bridge.

Crucially, that contract length would hurt the Blues as despite him being a regular with the west London outfit for that five-year period, he would never sign a contract extension and was therefore free to leave last summer when Real Madrid came calling.

During his time at Stamford Bridge, Rudiger would make 203 appearances, contributing 12 goals and seven assists, winning an FA Cup and Champions League with the Blues.

Reports from Germany had suggested that the Germany international was keen to extend his stay at Chelsea and had agreed to a new deal to ensure his contract went beyond 2022 but the Blues were unable to finalise his extension due to the financial sanctions that were facing former owner Roman Abramovich.

Wage demands were also reportedly a sticking point in negotiations, but the Blues clearly left it too late when trying to extend his deal, as the German defender was a consistent force during his time in England, averaging above a 6.7 rating from WhoScored in all of his seasons as a Chelsea player.

Back in 2021, Rudiger earned a "world-class" label from former Bayern Munich defender Jerome Boateng as a result of his strong performances for club and country.

He said: "For me, he is currently the best German defender – and definitely among the top five in his position in Europe."

Although Carlo Ancelotti's side could lose out on the La Liga title this season, they look a strong bet for the Champions League once again and will face up against Graham Potter's side in the quarter-finals next month.

It would not be a surprise to see Rudiger return at his very best, as the £247k-per-week defender has averaged a superb 6.92 rating from WhoScored across his six appearances in the Champions League so far this campaign, with the most aerials won per game in Los Blancos' squad.

Unsurprisingly, as he is playing for one of the top clubs in Europe, Rudiger's market value is still very high, with Football Transfers rating him at €47.4m (£42m).

This emphasises that no matter how much money Todd Boehly spends at Chelsea, the club still needs to do better in keeping their top players for the long term, as Rudiger would have been a big help to Potter this season.

Indeed, to lose him was one thing, but to see him walk away for free must be gut-wrenching.