The 24-year-old forward has made just three appearances for the Bernabeu club since completing his world-record £86million move from Tottenham in August.
Bale is currently sidelined with a back injury and, earlier this week, Real denied reports that the Welshman had suffered a slipped disc and required surgery.
Coleman believes Bale's lack of a proper pre-season is a reason for his injury-troubled start to life at Real, and believes it will take some time for him to produce the sort of thrilling performances which were his trademark in the Premier League.
"Anyone who plays top level professional football and you don't have a pre-season, if you expect to start any game, you are bonkers, crackers. It doesn't happen," he said.
"If you think you can just pick up where you left off the season before, that doesn't happen either. It takes time.
"It doesn't matter how good you are - and he's a super player and a super athlete. You can't just switch it on or off if you've missed all that pre-season.
"You are always playing catch-up so I wouldn't be surprised if it's after Christmas before we see Gareth getting into it again.
"It may take him some time. He's in a new environment that he needs to get used to and he's playing catch-up all the time.
"I hope it's not serious but it may be some time before we see Gareth doing what he is capable of."
Coleman also dismissed suggestions Bale has been carrying a back injury for some time and that he aggravated it on international duty.
"There's been no problem with his back with Wales - I think he missed the Olympics with his back and our friendly against Mexico that summer," added Coleman.
"But he's never complained about his back with us."
You can watch Belgium v Wales on Sky Sports 3 HD, from 7.55pm