Ole Gunnar Solskjaer admits he should have taken Fred off at half-time in Manchester United's Champions League defeat to Paris Saint-Germain.
Fred only received a yellow card for headbutting PSG midfielder Leandro Paredes in the first-half and also appeared to stamp on the Argentinian in a separate challenge.
However, a minute after Marquinhos restored PSG's lead in the 69th minute Fred received a second yellow card when he played the the ball in a tackle on Ander Herrera.
Donny van de Beek and Paul Pogba warmed up in the first-half and Solskjaer considered withdrawing Fred at the interval but kept the Brazilian on.
"It's not a foul," Solskjaer said of Fred's second yellow. "Fred played really disciplined in the second-half and when he makes a tackle like this you can't really fault the boy.
"We said at half-time just be sensible and he was, he played well. When the decision the referee made you maybe look back but there was no reason from his performance to take him off.
"He was lucky to stay on in the first-half but the referee made a mistake there, that's allowed. But Fred knows he shouldn't put his head towards someone even though he doesn't touch him.
"You can't say Ander tried to provoke him, that's a bit unfair to Ander. I thought Fred kept his head and did well in the second-half and he was unlucky with that decision by the referee. You can say in hindsight to have 11 men would have been better.
"We spoke to him at half-time and to be sensible and I thought he was. I thought he played really well, he was very sensible in the second-half and he covers the ground, gets up to men without trying to tackle.
"The tackle he gets a yellow card for is nowhere near a foul, if you see that again it's a mistake by the referee. But human error is allowed and I can't complain about that now. Fred was unlucky there but he was probably lucky not to come off in the first-half."
In a separate interview with Viasport, Solskajer said: "The second yellow from Fred was not yellow. But maybe he should have had red before. I must maybe hold my hands up, I should probably have taken him off before."
