The Wales boss fears a lack of preseason training means the world-record signing faces a tough start to life at the Santiago Bernabeu and may not hit top form until next year.
Since his arrival, the Welsh winger has only played three games and has been sidelined in recent matches with a thigh injury and a reported back problem.
The 24-year-old will miss Wales' final World Cup qualifier against Belgium on Tuesday and Coleman has warned that the star may not hit his stride until after Christmas.
"I took a lot of criticism in the last camp against Macedonia and Serbia for not playing Gareth. Why would I not play him if he was fit?" Coleman told reporters. "He had no preseason training. Anyone who has played top level professional football will know that if you have no preseason and expect to pick up where you finished the season before then you are bonkers and crackers.
"It doesn’t happen. You can’t miss a whole preseason. It takes time. He’s a super player and athlete, but you can’t just switch it on and off. It doesn’t matter how good you are. When you have missed all of preseason then you are always playing catch up."
The Group A clash at King Baudouin Stadium – which will be Craig Bellamy’s final game before retirement – could also be Coleman’s final game in charge following a failure to agree a new contract.
"It's a big decision and I'm torn," he said. "I'd love to be the man to lead us to a major finals but the unrealistic expectations and frustration of not having all the players makes me think, 'Do I want that again?'
"It's the big carrot that is dangling ahead of me."