The Russians were beaten with two minutes left on Sunday as their chances of progressing to the last 16 were narrowed.
"We knew that Belgium were favourites, but that wasn't obvious during the game," Fayzulin told FIFA.com.
"We managed to put them under pressure and had several good spells, but we simply couldn't score. That's football, if you don't score you're in trouble."
Russia must now beat Algeria on Thursday to make it into the second round of the tournament.