Bayern Munich fell behind to Patrice Evra's wondergoal but hit back less than a minute later. The Dutchman felt his side needed a wake-up call.
The defending European champions booked their place in a third consecutive semifinal with a 3-1 victory in Munich on Wednesday, progressing 4-2 on aggregate.
After controlling possession throughout the first half the hosts were shocked into life when Patrice Evra's thunderous left-foot drive put United ahead.
Bayern immediately responded with a goal 22 seconds after the restart through Mario Mandzukic, before Thomas Muller and Robben finished the job.
"We played well in the first half," the Netherlands international told reporters. "(But) the first 10 to 15 minutes of the second half was a disaster.
"We were so slow and you can't do that in the Champions League because you will be punished - and we were.
"The goal was a wake-up call, because we went on to score three goals."
Fellow goalscorer Muller felt the Bundesliga champions had done enough to progress to the last four, and praised the way they responded to Evra's strike.
"It was a very intense evening," he said. "Thankfully we managed to score the equalizer right after Manchester took the lead.
"I think we deserved to progress.
"It took some patience but thankfully we managed to do enough in the end."
Bayern is joined in the semifinals of this season's competition by Atletico Madrid after it eliminated Barcelona, while Chelsea and Real Madrid booked their places on Tuesday.