The Sweden striker got a straight red card after lunging at fullback Andres Guardado with his studs up but Ancelotti said he felt it was harsh on the volatile Ibrahimovic who suffered his third sending off in Europe's elite club competition.
"I did not understand it," Ancelotti told a news conference.
"He was out on the wing, he made a tackle and he did not deserve to be sent off," the Italian added. "That's football."
Ancelotti said he was satisfied with the result despite Valencia's 90th-minute goal giving the La Liga club a sliver of hope for the return leg in Paris in three weeks' time, when the talismanic Ibrahimovic will be suspended.
He dismissed suggestions his players had eased off towards the end of the game.
"I don't think we relaxed and we could even have scored a third on a couple of occasions. The team played a good match, we controlled the ball well and I'm pleased with our performance.
"Valencia showed their quality, we defended and worked well. We won the ball back well and made some swift counter-attacks."
PSG defender Mamadou Sakho said the Ligue 1 leaders were upset at conceding late on after establishing a 2-0 half-time lead and squandering several chances to score a third goal in the second period at the Mestalla.
"We're disappointed to concede a goal, especially at the end of the game," he said on UEFA's website. "But overall, it's a good result. To come here and win is very good after all."
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