Jose Mourinho will be cross-examined as part of former Chelsea doctor Eva Carneiro's employment tribunal.
Carneiro has claimed constructive dismissal against Chelsea, having left the club in September following a row with Mourinho on the opening day of last season.
The doctor and physiotherapist Jon Fearn were withdrawn from first-team action after entering the field to treat Eden Hazard late in the 1-1 draw with Swansea City - infuriating then-Chelsea manager Mourinho as his team was reduced to nine players as a result.
Mourinho labelled Carneiro and Fearn "naive and impulsive" and the former did not appear on the bench in subsequent games before leaving the club.
The Portuguese, who was sacked by Chelsea in December following the club's dismal start to the season, was cleared of using discriminatory language towards Carneiro by the Football Association but still faces a personal legal claim for alleged victimisation and discrimination lodged by the medic.
Carneiro's case began at the London South Employment Tribunal on Monday, with QC Mary O'Rourke confirming that Manchester United manager Mourinho would be cross-examined - as well as Chelsea officials Bruce Buck and Marina Granovskaia.
A Portuguese insult allegedly used by Mourinho could be central to the case. A short piece of a statement from Mourinho given in court defended his language, with both prosecution and defence poised to call expert witnesses in an attempt to determine exactly what was said.
Mourinho's appearance will be hotly anticipated, though the case threatens to overshadow his start to life as United boss, having replaced Louis van Gaal in the Old Trafford hot seat last month.
