Rosenior was paticularily unhappy that after going a goal behind his side failed to take chances which came their way,
"The disappointing thing with the performance was it was an even game and we had the better of the chances in the first half, but we didn't show any conviction or even hit the target." said the bees manager.
With changes expected to be made for Tuesday nights trip to Sixfields the Brentford manager has limited options after sending Darius Charles, Charlie Ide and Alex Rhodes all away on loan.
Oli Skulason, Andy Frampton and Kevin O'Connor are likely to feature again as the manager looks to get them games and have the experienced trio back to full fitness. The bees manager believes that his side need to raise their game an awful lot if they are to progress to round two,
"On Saturday, Rotherham were stronger than us and quicker to the ball than us in the second half. As soon as we conceeded, we should have lifted ourselves but too many players went under and on Tuesday I don't want that to happen."
"It's unacceptable and we need to do something about it"