Morgan Schneiderlin has asked to leave Manchester United, with West Bromwich Albion leading the race to sign the midfielder after lodging a £13 million bid.
Schneiderlin, a £24 million signing from Southampton in July 2015, has failed to start a Premier League game for United this season.
Everton manager Ronald Koeman is also keen to sign Schneiderlin, having worked with him at Southampton.
"He is a great professional, a fantastic boy, very honest and open," Mourinho said. "He opened his heart to me a couple of times and my answer is simple.
"If he was playing regularly with me, I have the right to say no way. But if he is not playing, I have no right to say you are going nowhere.
"My answer was that, if the offer is right and the board thinks it's a good offer in relation to quality of a very good player like Morgan, I wouldn't stop him going."