Mariappa, who was making his first appearance of the season having returned to the club during the summer, was among those to struggle against Millwall strikers Lee Gregory and Steve Morison, who scored the winner.
"You know what kind of game it's going to be when you come here, and we didn't stand up to it," he told the club's official website. "They pretty much bullied us, and deserved to beat us.
"They defended very well. I'm not going to use the pitch and things like that as an excuse. You can't just turn up on the day and expect to beat people; you have to earn it and we didn't and they did.
"I've been waiting a long time, and for whatever reason hadn't had the opportunity, and then had the injury. It's nice to get back on the pitch, but unfortunately it wasn't the result or performance I'd wanted to come back to."