The Spanish full-back was dismissed for a second bookable offence in the 81st minute of Saturday's Premier League goalless draw - the Welsh club's second red card in as many top-flight matches after Wilfried Bony received his marching orders against Southampton last weekend.
Rangel's first booking came after the long-serving defender threw the ball away, before a collision with Will Buckley led to his second, and Monk has moved to criticise the standard of refereeing in Swansea's recent fixtures.
"The first one - obviously, if you throw the ball away, the letter of the law is that it's a yellow card, so no problems," he said. "The second one was 50-50 for me.
"Sometimes the ref won't book you for that, but I think it was very clear today the referee was always going to book you for something like that.
"It just worries me that the last few games - if you look at Everton [3-0 League Cup win on Wednesday] and Southampton - we just seem to be getting punished for every single tackle.
"Every single body contact we seem to get punished with a yellow card and then I look at the other team doing the same and it seems to go unpunished.
"That's something we'll have to have a look at.
Monk also suggested Foy may have been influenced by the home fans at the Stadium of Light.
"You always run the risk when you're on a yellow card, but you can just sense it," he added. "The crowd get up a little bit, especially away from home, and they start getting on the referee's back every single decision.
"Then the ref comes under pressure. Good refs deal with it, other refs don't."
