By Dan Marsh Manchester United are unwilling to sell Donny van de Beek to a Premier League rival this summer, according to the player’s agent. The Dutch midfielder, 27, has endured a horrific time since joining the Red Devils in a £40million deal from Ajax back in 2020 having hardly played under a host of different managers, including Erik ten Hag. Van de Beek was officially deemed surplus to requirements at the start of the season but was unable to agree a move away. He subsequently joined Eintracht Frankfurt on loan in January, but the Bundesliga outfit have no interest in signing him perman…