Antonio Conte’s side were without striker Alvaro Morata and midfielder N’Golo Kante, while wing-back Victor Moses limped off with a hamstring problem.
The majority of the squad had also been away on international duty, meaning there was little time to prepare for the trip to Selhurst Park.
But goalkeeper Courtois said: “The international break is no excuse. We had some injuries, we missed some important players, but that doesn’t explain everything.
“We lacked intensity and also our way of football. They started well. The first goal [an own goal from Cesar Azpilicueta] was just bad luck. A ball that bounced from a knee. It was probably just what they needed to get the match in their hands, but we just haven’t played our game.
“The second goal was too easy too. Loss of possession, a player [Wilfried Zaha] who could run at two defenders.
“In the second half, the game just wasn’t going our way. We weren’t intense enough. Not at our best level.
“We just have to keep on working. We have an important game on Wednesday against Roma and some important ones after too.
“We wanted to win against Palace to stay in touch with the top teams in the table. We have to make sure that we start our next games better than we did at Palace.”
Chelsea are already nine points behind League leaders Manchester City having lost three of their first eight games.
The defending champions bounced back from a disappointing start last term by going on a club record 13‑game winning streak to put themselves in control of the title race.
Captain Gary Cahill believes they need a similar streak to get back into contention.
He said: “Manchester City are blowing teams away. They are in a rich vein of form, we know what that’s like and to have the confidence of winning and winning and winning. So we need to go on a run. Like last season? Yes, potentially. If you can be in and around the top by January time, then it’s a decent position.
“You saw the real Chelsea last season and it is important we try and emulate that sort of form this season. It’s about consistency.”