Mourinho, who has never won at St James' Park as a manager, had his run extended as Matt Richie's goal in the second half was the only difference between the sides.
The Portuguese manager admitted that Richie's goal was a "mistake", but also said that United never looked like scoring in the contest.
"We could be here 10 hours and we wouldn't score a goal," Mourinho told MUTV. "That's the feeling. Anthony [Martial] in the first and in the second half Alexis [Sanchez] with an open goal. Then we have situations.
"The goal is a mistake. [From a] long-distance free kick, we lose the first ball in the air because we didn't compete and then we lost the second ball. Newcastle fight. I loved the way Newcastle fought for 90 minutes and I loved the way my players fought for the last 25 minutes to change the result."
United face Huddersfield Town in the FA Cup in their next game, before meeting Sevilla in the last 16 of the Champions League.