According to talkSPORT, the Portuguese side have slapped the huge price tag on the Belgian international as they look to ward off interest from the Premier League pair.
The talented midfielder caught the eye as Benfica reached the quarter-finals of the Champions League last season, only to lose to Chelsea.
And Roberto Di Matteo is now keen to bring the former-Standard Liege player to west London, although Sir Alex Ferguson is also an admirer of the 23-year-old.
But Benfica president Luis Filipe Vieira warned the two parties that they will have dig deep into their pockets and pay the full release clause written into Witsel's contract if they want to land the midfielder.
"Would we sell Witsel?" Vieira told radio station Antenna 1. "Only if someone pays his release clause.
"We know the clubs that are interested. They would have to pay a lot for him to go, but it doesn't come into our heads to sell him."
The 6ft 1in midfielder joined Benfica from Liege last year, and was one of his side's best performers. Should the Benfica star join Chelsea he would be the latest Belgium international to sign for a Premier League club, after Eden Hazard, Moussa Dembele, Thomas Vermaelen, and Marouane Felliani.
Manchester United have been linked with a number central midfielders this summer, including Joao Moutinho, Luka Modric and Cheick Tiote. But having been priced out of a move for Modric, Sir Alex Ferguson may feel Witsel's price tag is similarly excessive.