Mourinho was shown a yellow card by referee Mike Dean in the 77th minute after appearing to take a look at the tactical notes being prepared by Southampton's coaches:
While Mourinho accepted the punishment, he didn't express much in the way of contrition when attempting to excuse his actions:
Mourinho felt aggrieved by several of Dean's decisions, notably ruling out an apparent equaliser from Harry Kane for offside. He also believed Spurs were denied a penalty when Dele Alli looked to have been felled in the box by Southampton goalkeeper Alex McCarthy shortly before the break.
The Spurs chief pulled no punches in his assessment, per Football.London's Rob Guest and Joe Doyle: "For me, at this moment, the referees are not the referees. VAR should be called video referees. Our goal I also don't know but I confess that I didn't watch it yet. What I know is that the Dele Alli penalty was a penalty."
Mourinho's frustration was inevitable on what proved to be a tough day for Spurs. Not only did the Lilywhites lose ground in the race to finish fourth and secure the league's last UEFA Champions League place, but they also lost key players to injury.
The most costly injury involved attacking talisman Kane, who was withdrawn on 73 minutes with an apparent hamstring problem. Speaking about the extent of the issue, Mourinho was unable to offer definite details, although the initial situation appeared bleak:
Being without Kane for a prolonged period would create a huge problem at the heart of the Tottenham forward line, yet Mourinho would be confident about having the personnel to find a solution thanks to Heung-Min Son (who returns from suspension in their next match against Liverpool), creative midfielder Alli and versatile winger Lucas Moura.
However, the greater concern for Spurs is how often they are conceding goals. Southampton's winner came in the 17th minute, courtesy of Danny Ings, who flicked the ball over Toby Alderweireld before slotting in coolly.
The goal is the 13th that Mourinho's men have conceded in nine league games since he replaced Mauricio Pochettino. Familiar issues at the back have seen a dire run away from home continue:
A lack of credible protection from midfield hasn't helped a struggling defence. Tottenham's engine room has been waiting on the power and skill of club-record signing Tanguy Ndombele to make a difference.
Yet the former Lyon man who moved to north London for an initial £53.8 million in the summer has struggled with injuries. Ndombele's latest problem saw him substituted for Giovani Lo Celso after just 25 minutes against the Saints, much to Mourinho's frustration:
While Mourinho can lament Ndombele being unable to build some momentum, his comments could further shake the player's confidence. Mourinho admitted Ndombele told him "he was not feeling in condition to play" during December's 2-1 win over Brighton & Hove Albion, per BBC Sport.
Ndombele asked to be left out against the Seagulls, and his struggle for fitness and confidence is just one of the issues mounting for Mourinho. Kane's status and vulnerability at the back will also dominate the thoughts of a manager going through his first trying period since taking charge.