Leading 2-0 from the first leg at Old Trafford three weeks ago, PSG crashed out in remarkable fashion as Manchester United wrapped up a 3-1 win in Paris.
Romelu Lukaku scored either side of Juan Bernat's effort in a frantic first half that made way for an edgy second period at the Parc des Princes, where the video assistant referee's late intervention proved the catalyst to a remarkable win.
Marcus Rashford slammed home a stoppage-time spot-kick after referee Damir Skomina adjudged Presnel Kimpembe to have handled as United ran out 3-1 victors to see them through to the quarter-finals on away goals after it ended 3-3 on aggregate.
"It was a big decision, a cruel decision... but it's difficult," Tuchel said.
"There are reasons he can give the decision. It's always soft facts with hands," he told a news conference. "The shot is not a big, big chance. It's wide, then suddenly it's a penalty."
"I'm a big supporter of VAR, and I am still a big supporter of VAR," he said. "With hand, it's a super-difficult thing, because there are too many points in do we punish it or not punish it, distance, natural position... so many things to discuss. I think [it's] a 50-50 decision."