Right-back Simpson, an ever-present for Rangers so far this season, has sustained a small fracture in his back.
Manager Harry Redknapp said: "It's a big blow. Danny has played every game for us this season but he's picked up a back problem in recent weeks and it's got worse.
"He will be out for a while which is a blow to us, because he's been great since signing in the summer."
German-born Ehmer, 21, joined Carlisle at the beginning of November and made 14 appearances.
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- Harry Redknapp