The incident occurred in the second half when Mourinho ended up in a heated argument with goalkeeping coach Andrew Sparkes in the opposing team's technical area after Harry Kane's goal was disallowed for offside.
He then had a sneaky peek into a piece of paper on the Southampton bench, allegedly spying on their strategy, which prompted referee Mike Dean to show the Tottenham boss a yellow card.
"I was rude. But I was rude with an idiot. I clearly deserved the yellow card. I had bad words with the guy", he told journalists after the game, adding "There was of course a reason for it. I will not tell you but for some reason I had that reaction".