The attack-minded Portugal left-back has been linked with a summer move to the North-West rivals, with Real Madrid, AC Milan and Bayern Munich also reported to be interested.
But Benfica are unwilling to allow Coentrao to leave for anything less than £26million.
The club's president, Luis Filipe Vieira, told Portuguese media: "Coentrao can only leave if a club pays 30m euros, which is his value in the market."
Portuguese newspaper Correio da Manha has reported that Liverpool manager Kenny Dalglish has already agreed a deal for Coentrao.
But Real and Manchester City would have the financial clout to outbid the Anfield outfit if it becomes clear that the 23-year-old is available.