Was Reggie Nelson's Hit on Le'Veon Bell Dirty?

With 6:38 left to go in the third quarter of the Pittsburgh Steelers' win over the Cincinnati Bengals on Sunday night, Pittsburgh feature back Le'Veon Bell was knocked out of the game after a hit by Bengals safety Reggie Nelson in the area of his knee. 

It was an injury that left the Steel City holding its collective breath, as the Steelers prepare to take on the Baltimore Ravens in a Wild Card Game next week. Bell has been a dynamic double-threat this season, and he will be crucial to Pittsburgh's playoff run. 

You can see the play again in the GIF below, in which Nelson dives helmet-first at Bell's right leg to make the tackle:

In his postgame press conference, Steelers head coach Mike Tomlin said that Bell hyperextended his knee, and that his availability for next week's game is unknown. 

This isn't the first time this season Nelson has had a questionable hit.

In Week 5, when Cincinnati faced the New England Patriots, Nelson went low on tight end Rob Gronkowski.

Since the NFL revised its rules on tackling in 2013 to come down harder on hits to the head and neck, defenders have responded accordingly—going low, often at the knees, and often resulting in injuries like the one Bell sustained on Sunday. 

When he spoke about the play in his postgame press conference, Tomlin indicated that he thought the hit was legal, though he refrained from sharing his "personal opinion" about the play.  

But "legal" and "dirty" aren't mutually exclusive. Even if Nelson's hit was legal by the letter of the rule book, was it a genuinely clean hit, or does it fall into an unspoken gray area players generally shy away from crossing?