Luke Berry missed an early penalty for U's, before Doncaster went 3-0 ahead.
Goals from George Maris and Berry closed the deficit, but Rovers won 3-2 - their first victory in six matches.
"Against the top teams I bang the drum about taking your chances. In one box we weren't ruthless enough, and in the other we weren't good enough," he said.
"I see chances that have gone begging. And the other side of the game, the defensive side, it's three goals that we have conceded that are bargain basement."
Derry's squad has been hit by injuries all season, and Cambridge were without forwards Uche Ikpeazu and Barry Corr for the visit of Darren Ferguson's side.
Ben Williamson started the game up front for the U's, but failed to add to his one goal from 38 matches this season.
"We've missed some big players at key moments of our season. Against Doncaster we've missed Uche and Barry Corr and we're reliant upon a striker who's absolutely off form," said Derry.
"I'm not going to shy away from that. I see him every day in training, his character and application is exactly the same as last season, but the goals aren't there for him at the moment."
Cambridge United manager Shaun Derry was frustrated after seeing his side concede three "bargain basement goals" against League Two leaders Doncaster.