NFL - Unnecessary Roughness

[Blue37]

Titans - Panthers

Panther QB was flushed and rolled right. On the opposite side of the field, the Panther wideout was trotting across the field and was blindsided by the Titan CB. The act was penalized and the Titan coach was very animated protesting the call. The hit seemed otherwise legal, but was definitely unnecessary. I would have flagged it in a heartbeat on a Friday.

I cannot find the NFL rules online, just a "digest" that summarizes the basics. What is the NFL rule for unnecessary roughness? The R's explanation included "while the pass was in flight". Does that matter?