Second half goals from Billy Sharp and Dean Shiels gave the home side an unexpected 2-0 success at The Keepmoat Stadium on Saturday afternoon.
"We're very disappointed with both goals," the Rangers boss said after the match.
"We were architects of our own downfall in some ways.
"I thought it was an even game, but at times we looked lethargic, as if we had had two weeks off.
"We looked like we had lads who had been reading nice things about themselves.
"We had opportunities and moments, but Doncaster are a good side and we didn't take our chances.
"For the last half-hour it was as poor as they have played for me but I can't be too hard on them because they have been very good until now."