The Giallorossi were 3-0 up through a Tammy Abraham brace and sensational Mayoral back-heel flick, but put their feet up and conceded two goals in the closing stages.
They were able to win the group because Zorya held Bodo/Glimt to a surprise 1-1 draw.
“We cannot keep creating problems for ourselves,” an irritated Mourinho told Sky Sport Italia.
“It is not possible that we are winning 2-0 for 45 minutes, then lose the ball repeatedly and sit too deep. The two goals we conceded are unacceptable.
“I am only happy because we won and Zorya did their duty, but that is the only thing that makes me happy tonight.”
There was also Mayoral’s first goal of the season, a wonderful back-heel flick.
“He was one of the few positives. His goal was beautiful, but I also liked the work he put in. I am not surprised, because he always works hard.”
Winning the group sends Roma straight through to the Conference League Round of 16, avoiding a two-legged play-off in February.
“We don’t have the squad to play an extra two games. We were fortunate, because that extra round really would’ve been something we didn’t need.”
What a lovely finish from Roma’s Borja Mayoral! ✨
— Football on BT Sport (@btsportfootball) December 9, 2021