The Serbian midfielder was sent off late on as the Red Devils hung on to a 2-1 lead at Vicarage Road.
A second yellow card was flashed in his direction during stoppage-time, with an ill-advised barge on Will Hughes forcing him to head for the dressing room ahead of his team-mates.
Matic will now serve a one-match ban, with the 30-year-old looking forward to a “rest”, but there could be more repercussions to come as he prepares to face his manager away from the public eye at training.
Quizzed after a narrow win at Watford as to how Mourinho had reacted to his dismissal, Matic said: "We will see on Monday!
"I think the first yellow card was a clear yellow card. The second one, I'm not sure. It was a foul but I'm not sure if it was a yellow card. It's OK. I accept the decision of the referee and I will have one game to rest."
Romelu Lukaku and Chris Smalling had put United in charge against Watford, but Matic admits the Red Devils ended up making life difficult for themselves against a side that had opened the 2018-19 campaign with four straight victories.
He added on Mourinho’s words of advice at half-time: "He said we had to concentrate and that we could not allow them to create the chances because if they score a goal we are in trouble.
"We knew that. But they have quality, they have confidence because they won the first four games. They scored and after that it was difficult for us. We had some counter attacks to finish the game but we didn't score. We are happy. We are happy for this win."
Smalling proved to the unlikely hero for United, with his goal seven minutes before the break proving to the difference, with Matic quick to talk up the qualities of a defender who was snubbed by England over the summer and has often struggled to win over his detractors.
"I was surprised that he did not go to the World Cup because he has played for Manchester United regularly," added Matic.
"I was surprised but it was the decision of the coach of course.
"Chris is great. As you see, in the box he is always dangerous. Last season he scored some important goals for us. "