Janmaat has admitted that his actions probably cost the Magpies any chance of getting a win at Swansea.
The Dutch defender was sent off before half-time as Newcastle slipped to a 2-0 Premier League defeat.
“It was stupid of me and I have to apologise to my team-mates, the supporters and everybody, because trying to win a game with 10 men is very difficult,” Janmaat told Newcastle’s official website.
“The first yellow card was clear. The second one, I lost the ball, he (Montero) ran past me and in a split-second I touched him.
“I pulled him back and that was stupid.
“With 11 men at Swansea it is already hard but with 10 men it makes it almost impossible. We didn’t give up though, and the rest of the team gave it everything, but it was a disappointing game.”