I'm generally of the opinion that O'Nien would be fine there. His engine is perfectly capable. While he's not lightning he'll run for the entire 90 perfectly easily. Not sold on Johnson as cover, personally. However, I think Crompton would do a decent job. Good on the ball, mobile. I'd be looking to him as emergency third choice.
Ultimately it comes down to resource allocation. How many of the next 21 games is Trai Hume likely to miss? He has been available for selection for virtually every game in the past three seasons so the evidence says he's pretty robust. It's a gamble, but it's probably not as much of a gamble as it initially looks.