Nigel Pearson brushed aside congratulations on reaching 500 games as a manager after seeing his Bristol City side beaten 2-0 by QPR.
Rangers were two up inside 22 minutes and never looked like surrendering the advantage against opponents who created little.
llias Chair escaped the attentions of Adrian Mariappa to head home Todd Kane's 11th-minute cross.
Pearson said: "It wasn't the way I wanted to mark my 500th game. I judge our performances against what we can produce, rather than how the opposition play.
"Since I arrived there have been three good displays, two of which brought wins, but today we slipped below the standards we set.
"We allowed Rangers to score their goals without having to work hard for them, which is not what we want. And we were too passive in the first half.
"Having to rejig the team so early on made the game more of a challenge, but that's football.
"It's important that when we cannot dominate games our collective work ethic and positive mentality gets us through. That wasn't the case today.
"The second half performance was honest and we had chances, but overall we allowed the game to somewhat pass us by.
"I am not going to blame injuries because the players selected have shown they are capable of playing a lot better.
"We now have a free week before our next game and I must use it to improve certain areas, while hoping some of our injured players become available.
"The players know how I feel about the game. We have discussed it – put it like that."
Nahki Wells wasted a great chance to reduce the arrears when firing wildly over after 68 minutes.
But Rangers could have won by a greater margin and manager Mark Warburton was understandably delighted.
"They did a great job in that respect and after a sloppy opening five minutes when we could have conceded, it was an excellent team performance.
"There are such fine lines between success and failure in the Championship, which is what makes it so exciting and why we all love it.
"We played really well at home to Bristol City this season and lost the game 2-1. But today we got what we deserved because we were good all over the park.
"It was our intention to get in a lot of early crosses and Ilias Chair has got on the end of one to put us in front.
"Rob Dickie is a very talented player. He relished his battle with Famara Diedhiou and came up with the second goal.
"All our back-line played well and we could have been 4-0 up at half time against a very good team.
"Sometimes we didn't get the final pass right, which is something to work on, but overall I was very happy with the way we played."