Manchester United have completed the Deadline Day signing of Porto left-back Alex Telles for a fee of £15.4m.
The 27-year-old has signed a contract until June 2024, with the option of a further year.
United's hopes of signing the left-back appeared to be fading after their offer was initially rejected by the Portuguese champions, who were holding out for £22.5m.
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However, negotiations began again on Sunday morning, with Porto changing their minds on United's most recent bid as they were convinced that no subsequent offer was likely from Old Trafford, with the transfer window about to close for Premier League clubs.
"To join a club with the prestige of Manchester United is a huge honour," said Telles. "You have to work hard to get to this moment in your career and now I am coming to this club, I can promise that I will give everything in my heart to be a success here.
"I won many trophies at FC Porto and I want to continue that at United. The manager has a clear plan and direction for this team and I can't wait to pull on the famous shirt."
Telles signed for Porto from Galatasaray in 2016 and went on to win two Portuguese league titles and the Portuguese Cup in nearly 200 appearances for the club.
He becomes Ole Gunnar Solskjaer's second signing of the window, following Donny van de Beek's move from Ajax.
Solskjaer said: "He is a player we have been tracking for some time and his performances over the past few years are exactly what we are looking for.
"He is a fighter and a winner and will add real determination and competition to the squad.
"Alex has the qualities, both as a player and as a person, that we want here at Manchester United."
Telles will fly to Sao Paulo to join the Brazil national team ahead of World Cup qualifiers against Bolivia and Peru.
Diallo to join Man Utd in January
Atalanta winger Amad Diallo will join Manchester United in January.
United reached an agreement with the Serie A club for Diallo, who has been on the radar of United scouts since 2016, when he was spotted playing at U15 level.
United will pay around €20m plus €21m in add-ons (£37.2m) for the 18-year-old, who looked set to join Parma until the deal collapsed.
The transfer is subject to Diallo passing a medical, agreeing personal terms and being granted a work permit.
Diallo made the move to Atalanta's academy in 2015 and was handed his senior debut last October in the 7-1 victory over Udinese.
The youngster found the net after coming off the bench in that game and in doing so he became the first player born in 2002 to score a goal in Serie A.
Analysis: Telles adds attacking threat - but what about defensively?
Sky Sports' Ron Walker:
It doesn't take long watching Telles in action to see his main quality strikes a similar chime to that of many Brazilian full-backs, with his productivity on the ball a major asset to Porto in recent years, registering some 54 assists across his five years at the club.
While United's main defensive problems may lie in the heart of their defence, there is certainly a strong case to be made about how the Brazilian can improve their backline.
While Luke Shaw and Brandon Williams, the club's two regular left-backs in 2019/20, failed to register a single assist between them across the campaign, Telles was busy making eight goals for his Porto team-mates in the Primeira Liga, which may come as little surprise from a man who attempted almost 10 crosses per 90 minutes on average, and completed around a quarter.
Much may be said about his goal tally, too, as he found the back of the net 11 times last season, but with eight of those coming from the spot, fellow Portugal import Bruno Fernandes may have a thing or two to say about that at Old Trafford.
"He is very Brazilian in his approach," Vitor Goncalves of Portuguese national broadcaster RTP told Sky Sports. "His passing is very good - that's one thing that makes him different, even from other Brazilian left-backs."
In isolation, Telles' passing stats don't make remarkable reading, the fact he created 57 chances, the seventh-highest tally across the entire division, certainly does.
But just as Brazilian full-backs are known for their attacking prowess, so too has their defensive quality been called into action at times.
"He's not so good defensively," Goncalves adds. "But he's still a defender over a winger! He's a top Porto player, and the vice-captain behind Danilo Pereira."
If Manchester United are looking for a defender to shore up a backline which has already shipped 12 goals in its opening three games - a third of their tally for the whole of 2019/20 - Telles may not be the answer.
But with much of their problem during that season an inability to break teams down who played with a low block, he will add some quality from wide areas and a goal threat from the back which the club has undoubtedly been lacking.
Adding to the defensive options is timely for Manchester United, who were humiliated by Tottenham on Super Sunday.
This 6-1 defeat was only the second time United have let in that many in a Premier League game at Old Trafford. They had not let in four in the first half of a league match since 1957 before this one.
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Sky Sports' Gary Neville described the performance as "pathetic" but perhaps the most alarming thing is it had been coming. United's defence has been in disarray this season.
Crystal Palace's 3-1 win at this venue in United's first game of the season two weeks ago was a shock. Roy Hodgson's side were the second-lowest scorers last season, while United had let in just seven in their final 14 Premier League games of 2019/20.
Palace tore through the United defence and could have scored more.
A week on, Leandro Trossard hit the woodwork five times before United escaped Brighton with a last-gasp 3-2 win.
Victor Lindelof - inconsistent throughout his time at the club - lost his Premier League place at the heart of United's defence for the visit of Spurs, but the returning Eric Bailly fared little better alongside a badly-out-of-sorts Harry Maguire.
Sky Sports' Peter Smith looks at how a string of errors from Man Utd's £188m backline leaves them ranked among the Premier League's worst defences this season - read the full article here.
Dortmund: 'Door closed' on Sancho deal
The pursuit of Manchester United's main target during this window, Jadon Sancho, appears close to failure after Borussia Dortmund's sporting director Michael Zorc said on Saturday "the door is closed" on the move.
United failed to agree a deal for the 20-year-old before Dortmund's August 10 deadline, and the £91.3m offer they made this week was rejected by the German club.
However, one player who does look likely to be joining United is Edinson Cavani, who is close to signing a two-year deal.
The 33-year-old striker is a free agent after leaving Paris Saint-Germain at the end of last season, and is set to bring some valuable experience to United's youthful forward line.