Per Mike Puma of the New York Post, Conforto left the game after being hit in the head when he collided with Cano while they were chasing a pop-up.
Puma added Conforto will ride the train back to New York from Washington D.C. for further evaluation.
Nationals second baseman Howie Kendrick hit a pop-up down the right-field line in the bottom of the fifth inning. Conforto was running in to make a play when his face collided with Cano's shoulder, causing him to fall to the ground before leaving the field.
Coming off an inconsistent 2018, Conforto has been one of the Mets' most valuable players this season. The 26-year-old ranks second on the team with a .404 on-base percentage and .500 slugging percentage entering Thursday. He hit his ninth homer of the year in the third inning.
Keon Broxton, who has a .394 OPS in 33 games, took over for Conforto in right field.
The Mets entered the day with a 20-21 record, 3.5 games behind the Philadelphia Phillies in the National League East.