Forest headed to St James' Park looking to end their run of four consecutive defeats, but dealt a big blow with Matt Ritchie's early opener before Nicolao Dumitru-Cardosa’s first goal for the club got them back in the contest.
However Mill's two yellow cards in quick succession on the hour mark proved to be the difference, with Dwight Gayle grabbing the points with the brace and Montainer stated that it was the dismissal that was the difference.
"I prefer not to comment as everybody saw what happened," he said. "It is tough as it was a yellow card and no more."
"It was incredible and it changed the game," the coach stated. "We played a good first half and had a good reaction after an unlucky goal with some opportunities to score."
“If we stayed 11 against 11 we could have got a better result," Montanier added. "Quickly after we conceded the second goal and 10 against 11 was too tough."
Manage it well
It has been a busy festive period for the former European champions, and is expected to get tougher with their third game in a week on Monday against Barnsley and the coach stated that fresh legs will be brought in.
“We have to manage as we have three games during this week," he continued. "We know that some players cannot play two consecutive games so we had to change a little."
"We gave a good answer on the pitch with these players and others," he admitted. "I am happy that Nico scored the first goal as he works hard in training sessions and worked hard today and deserved to score."
Nottingham Forest will take on Barnsley at the City Ground on Monday, December 2 with kick-off at 3pm GMT.