The 22-year-old England international endured a dismal return to his former club as Nacer Chadli’s double and goals from Eric Dier and Emmanuel Adebayor completed a convincing win for the hosts.
This result sent the newly promoted Rs to the bottom of the Premier League and Caulker admits the team must raise their standards if they are to avoid a relegation struggle.
"It was a bitterly disappointing performance and the result proved that," said the summer signing from Cardiff.
"We were second to everything all over the pitch and we paid the price. We got off to a sloppy start and they got the goal and a bit of momentum.
"We know we have got to be better than that this year. We hope that will be the lowest point of the season. It was a poor, poor showing and there are no excuses.
"There is no hiding and we definitely have to go and be better against Sunderland at home. We have personal pride and it was not evident, we were weak all over the pitch.
"If we are going to stay in this league we are going to have to be a little bit stronger and more passionate at times."
Leroy Fer only joined from Norwich last week, but has echoed Caulker's call for more battling spirit.
"We didn't play to our abilities," said the Dutch midfielder.
"We have a strong team and what we showed was not the best. The goals in the first half were too easy. We had to fight and we didn't - It was a shock.
"We have to look forward to Wednesday (the Capital One Cup tie against Burton) and stick together. We have a great squad. Today wasn't a good game for me but I want to show more on Wednesday."