It has been a torrid 24 hours for Man Utd as on Sunday afternoon they were thrashed 6-1 by Jose Mourinho's Tottenham at Old Trafford.
Prior to that match they had only signed Donny van de Beek from Ajax, with fans clamouring for more reinforcements.
After that defeat by Spurs, the pressure on the United board to sign players on deadline day increased.
The Telles deal had been in the pipeline for a while, with Porto initially saying they would only allow their captain to leave should his release clause be paid.
However, they have settled for the initial fee above plus add-ons.
“To join a club with the prestige of Manchester United is a huge honour.," he told the official club website.
"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.”