Celtic manager Martin O'Neill admitted that Rangers had deserved the 2-0 victory that created a three-point gap at the top of the league.
"The first half we dominated," he said. "But Rangers were better than us in the second half and deserved it.
"Rab Douglas is obviously disappointed with the first goal and it was possibly a turning point.
"We obviously need to win our game in hand, but I always said nothing was going to be decided here."
Douglas was left red-faced after allowing Gregory Vignal's 25-yard strike to slip through his grasp and into the back of the net.
"I am disappointed of course, but it's just one of those things," said O'Neill.
"It was a big couple of minutes with their goalkeeper making a wonderful save from John Hartson just before.
"Rab has been playing very well and with a lot of confidence, but that is naturally a setback.
"In a big game like this, it was a poor goal from our view point, but I'm sure he'll recover."
O'Neill was also satisfied with the performance of debutant Craig Bellamy, despite the Wales striker passing up a great chance to score in the first half when he was sent through one-on-one on fellow new boy Ronald Waterreus in the Rangers goal.