The 23-year-old Belgium international began his career at Standard Liege before making the move to Portugal last summer, signing a five-year contract at the Stadium of Light.

The midfielder, who has a 40million Euro buy-out clause in his contract, caught the eye of Premier League clubs during his performances in the UEFA Champions League against both Manchester United and Chelsea.

United scout Martin Ferguson as well as Manchester City assistant David Platt were spotted at Benfica's 1-0 defeat against rivals Sporting on Monday, sparking rumours of a summer bid.

Investment

Rui Costa believes Witsel is good enough to play in Europe's biggest competitions but hopes he can keep hold of his star man for a few years yet.

"Witsel is ready for any championship," he said. "He is one of the few high level young players ready to play in a top club, although I hope he can continue with Benfica for a long time.

"I hope he could move to a side that allows him to continue his career path, though whoever wants to take him has to make a great investment.

"Him in Italy? Everything is possible and if he had a future in Serie A I would wish it would be at [former clubs] AC Milan or Fiorentina."