The Shrimps found themselves 2-0 down after just 16 minutes at Home Park, Conor Hourihane and Paris Cowan-Hall putting Argyle into a commanding position in what was John Sheridan's first game in charge.
Substitute Ryan Williams reduced the arrears for Morecambe but it was not enough and the visitors saw Andrew Parrish sent off late on.
"Indiscipline has cost us. They had a new manager coming in and the crowd were up for it so the last thing we needed to do was give them a two-goal headstart," Bentley said.
"We made some poor decisions, which cost us two goals. Credit to Plymouth, they took the two goals well but they did not have to work too hard to get them and that's what disappoints me. We gave ourselves a mountain to climb.
"Once we changed it around and made some tactical substitutions we took control and should have got something out of the game given the amount of possession we had and the chances we created.
"We scored a good goal but should have had more than one goal to show for all our work and possession.
"I must mention the Plymouth pitch - and the groundsman who has done a good job - because the pitch was perfect, one of the best we have played on. It meant we were able to play some good football."