I understand the dual speed threat of Cooper and a healthy Bell.
But if Bell plays, I think the main question is whether he's close enough to 100% to force the defense to respect that speed. If so, his mere presence does pose a problem for the ND defense. If he's 80%? Maybe not so much.
Where's the break even point -- 80%? 90%? More? Less? That's what Saban gets paid $5M a year to decide.