United reportedly sent scouts to watch Çalhanoğlu in the 4-4 draw against Roma in the Champions League against Roma last Tuesday.
The Bundesliga side took an early lead when Hakan Çalhanoğlu won his side a penalty on four minutes which Hernandez converted from the penalty spot.
Çalhanoğlu setup Hernandez with the second on 19 minutes and Bayer looked to be plain sailing until De Rossi scored a brace before the half time whistle.
Pjanic gave Roma the lead from a 54th minute set-piece and Falque looked to have put the game out of sight when he netted in the 73rd.
However, Bayer never gave up and Kampl gave his side a glimmer of hope when he scored with a brilliant striker six minutes from full-time and Mehmedi equalised just two minutes later for a dramatic end to the game.
Bayer reportedly want £28.9 million for the Turkish international star who has a contract with the Bundesliga outfit until 2019.