Sky Sports News are reporting that Fosu-Mensah is going through the procedure before joining up with Palace.
Fosu-Mensah, 19, can play centre-back in De Boer’s 3-4-3 formation or as a wing-back.
Mourinho wanted the teenager tied to a long-term contract and a new deal was signed last season that has an option to extend to 2021.
TFM also made a shock return from a shoulder injury to be part of United’s squad for the Europa League final win over his former club Ajax.
But Mourinho could not guarantee the youngster regular games this season and sanctioned a deal to Palace.
Fosu-Mensah’s impending Palace move means the club are likely to drop their interest in Arsenal’s Calum Chambers.