Mesar made his pro North American debut in Laval on Friday, finishing the night with two shots on goal and a minus-one differential in the Rocket's home opener.
The 18-year-old had also suited up for three preseason games with the Habs during training camp, registering a plus-two differential while averaging 12:53 of ice time.
Last season, Mesar picked up 16 points (8 goals, 8 assists) in 37 games with HK Poprad in the Slovak Extraliga. He was selected 26th overall by Montreal at the 2022 NHL Draft.