Watkins was shown a second yellow card for simulation late in Villa’s 3-1 home defeat to the Red Devils on Sunday.
But McGinn claims Watkins, who did not appeal for a penalty when going down under goalkeeper Dean Henderson’s challenge, was simply trying to protect himself from injury.
Instead McGinn believes the reaction from himself and the rest of Watkins’ Villa team-mates may have contributed to referee Chris Kavanagh’s decision.
“I think it was more me and the other lads claiming for a penalty,” said McGinn. “From where I was looking I thought there was contact but the referee decides it is a second yellow which I think is harsh.
“Anyone who knows Ollie’s character knows he is not that kind of player. He thinks there is going to be a coming together so he has to protect himself.”
Villa will now be without 13-goal top scorer Watkins, who had played every minute of the campaign prior to his dismissal, for Thursday’s visit of Everton.
A victory over the Toffees would see 11th placed Villa climb back into the top half of the Premier League table.
McGinn said: “We want to bounce back. We want to finish as high as we can and keep competing for places higher up the table in the top half. We have the chance to do that over the last four games of the season.
“The general feeling (in the dressing room) after Sunday was disappointment.
“We’re ambitious, we want to climb up the league, we want to be a proper Aston Villa team.
“We were aware that Villa haven’t beaten Man Utd at Villa Park since 1995 so we wanted to be the team to do that, unfortunately it wasn’t to be, but there certainly were opportunities to do that.”