Brett Pitman's first-half goal was enough to give the visitors all three points at the Keepmoat Stadium.
"It was very poor and disappointing, the standards that we set earlier in the season were not met today. The lack of energy and desire was not there today," the manager said in quotes posted on the club's Twitter feed.
"We huffed and puffed today without having any real quality. Maybe some players had a bit of complacency after recent good results, this will be put right for the next match."
Striker Chris Brown added: "It was disappointing, we didn't start very well. Today we slipped below standards, we all need to look at ourselves and improve."
Doncaster sit 18th in the Championship table.