Australian David Carney struggled at left-back in the weekend's 2-2 draw at BBVA Compass Stadium in the first leg, prompting speculation that Costa Rican international Miller could be used for the Red Bull Arena return bout.
But Petke insists that it would be careless to bring in a 28-year old who has not played since August because of an Achilles injury.
"He seems to be OK, but he has been out for God knows for how long," Petke said.
"It's not an easy thing just to throw someone like that into a game. We're figuring things out, whether he will be part of the 18 or what role he will play. But he looks pretty good and he's feeling pretty good."
However, Miller was of the belief that he could do the job if the club decided to use him in the crucial playoff.
"My fitness is all right. The injury I feel OK. After two months, it is not easy to come back and play 90 minutes," he said.
"So, we need to think about that if they decide to play me, I'm ready. I'm just waiting for the opportunity. We'll see what happens and see what they decide."
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