England stopper David James endured a miserable debut for the Robins as they were ripped apart by the newly-promoted Londoners in the second half.
A shell-shocked Coppell said: "I made it clear before the game that I would be as interested as anyone to see where we were because we have had to make so many changes in our pre-season games.
"The way the match went was an indication that we have an awful lot of work to do to be competitive in the Championship.
"I am not going to make injuries an excuse. We had 11 players out there raring to go, but they were not able to put in the required performance.
"It's always a concern when you lose a game 3-0. There are a lot of areas in which we can improve.
"David James has only trained with us for a week, yet he made one fantastic save, his distribution was good and he came for crosses well as you would expect."