He's a one man defense and PSG have enough attackers.
PSG lost an unlosable game against MU last year.
PSG didn't even need to defend as MU wasn't that much threatening. Just 3 dumb actions : Kehrer gifting a goal to Lukaku with a wonderful pass to the belgian striker, Buffon gifting another goal to Lukaku after an hopeless shot of Rashford, referee gifting a late game penalty kick after another hopeless shot that was going to the sky anyway.
I don't see how Gueye would have changed anything there.