The violation was determined to be the result of a misinterpretation of the policy's reporting requirements, a source told ESPN's Adam Caplan.
Sherman never missed a game last season. The Pro Bowl cornerback was listed on the injury report in 11 of the team's final 12 games (including the playoffs), but the knee injury was never listed. In 10 of those weeks, he missed practice time, but the Seahawks listed the reason as "non-injury related." In Week 12, Sherman was listed with an ankle injury.
The NFL's rule says, "If any player has a significant or noteworthy injury, it must be listed on the practice report, even if he fully participates in practice and the team expects that he will play in the team's next game."
In the event of any future violation, this current violation will be taken into account in determining discipline, the source told Caplan.
NFL Network first reported the news of the NFL's warning.
After the season, Seahawks coach Pete Carroll said in a 710 ESPN Seattle interview that Sherman "dealt with a significant knee [injury] the whole second half of the season."