Martinez, who requested a personal hearing, was punished by the Football Association after admitting a charge of misconduct following "a breach of FA Rule E3 concerning media comments which implied that the match referee and/or match officials in general are motivated by bias and/or brought the game into disrepute."
The Wigan manager said in the aftermath of a 4-0 defeat at Old Trafford that referee Michael Oliver did not treat both sides equally and suggested that match officials struggle to be impartial at Old Trafford.
"Michael Oliver is a very capable referee, who has been outstanding in many big games, but it was clear from the opening minutes that he was not going to measure both teams the same way," Martinez said.
"There were a couple of tackles by United players which, if they had been the other way round, would have been red cards. In that last challenge by (Danny) Welbeck, he had his foot over the ball as he went in on Franco Di Santo. It was completely reckless and dangerous, and under the current regulations, it should be a red card, whether you play for Wigan, Manchester United or anyone else.
"We had a player sent off here last season, and it was rescinded straight away. Two seasons ago, we had two players sent off. Now this. I just feel it is a difficult place for referees. They need to be strong, and they need to be very, very experienced. They need to know how to handle the occasion. It’s not about handling the game here, it’s the occasion.
"Welbeck should have been shown a straight red, and Paul Scholes had two bad challenges that were worthy of yellow cards. We had James McArthur booked for something very similar, yet Scholes got away with both. I cannot understand how those same challenges can be judged in different ways."
Martinez has also been warned about his future conduct.