Striker Maynard was shown a straight red card for a challenge on Matt Lowton in the first half and a miserable afternoon for the Robins was complete when Jordan Slew slammed home a superb late winner for relegation-threatened United.
Millen felt Maynard's dismissal was harsh and believed the game could have gone either way.
He said: "Where do you start with that? It could have gone either way, whichever team took their chance (would win) and they took theirs with a good finish."
He added: "At the time I felt he could send him because he slid in, but he got a slight touch of the ball.
"The wet pitch didn't help, he slid through the lad and just caught him, so we'll assess it on the way home."
Source: PA
Source: PA