Assuming Muslim Tim Tebow would be as high-profile as Christian Tim Tebow, they'd probably regard him with suspicion and contempt as he prostrates himself on the sidelines facing east, touching his forehead to the turf. And probably, in this alternate universe, some Salon columnist would be asking, "What if Tim Tebow Were Christian?" and he would be adored by the leftist establishment instead of slathered with its hatred.
Evidently, we're to regard Christians as hypocrites for not feeling warm-n'-fuzzy about Muslim athletes. Why should they, though? Christians worship Christ, not generic displays of piety. It's precisely this that makes them not hypocrites and their religion not merely the empty and insubstantial theatrics their secular enemies accuse them of being. Why should they feel anything but unease toward the practitioners of what they see as a false religion? No one reasonably expects anything else from Muslims.
Here's a better question: what if Tim Tebow were Christian but America was Muslim? Would the social reaction to him be analagous if the roles were reversed and it was Christian America reflecting on Muslim Tebow? I think we all know the answer to this.