Chelsea have signed World Cup winner Enzo Fernandez from Benfica for a British-record 121 million euros ($131 million), the Portuguese club announced early on Wednesday.
The midfielder, 22, was named Best Young Player of the World Cup for his displays during Argentina's successful campaign in Qatar.
The £106.8 million fee eclipses the previous record of £100 million that Manchester City paid Aston Villa for Jack Grealish in 2021.
Fernandez only joined the Portuguese giants in July last year, for a reported fee of around £10 million.
He has signed an eight-and-half-year deal that will run until 2031.
Todd Boehly's consortium has spent hundreds of millions since buying Chelsea at the end of last season in the hope that his extra splurge will lift the club from 10th place in the Premier League.
Man Utd signs Sabitzer
Manchester United announced they had signed Bayern Munich midfielder Marcel Sabitzer on loan in the early hours of Wednesday.
United were forced into deadline-day action on Tuesday following an ankle injury to Christian Eriksen, which is set to rule the Denmark midfielder out until early May.
Manager Erik ten Hag completed the loan signing of Netherlands forward Wout Weghorst earlier in the January transfer window.
Sabitzer, 28, who was previously club captain of RB Leipzig, has won 68 caps for Austria.
"Sometimes in life you have to make quick and important decisions," said Sabitzer, who will be at Old Trafford for the rest of the season.
"From the moment I heard about this opportunity I knew it was right for me. I am a competitive player -- I want to win and help the club achieve its aims this season.
"I feel that I am at my peak as a player, and that I can contribute a lot of experience and energy to the squad."
Manchester United football director John Murtough said: "Marcel is a player that we have watched for a long time. The opportunity arose quickly, and we knew that he was someone with the ability and character to make an impact."
United are fourth in the Premier League and are hot favourites to reach next month's League Cup final.