IM not so sure if it is legal. From an article i found on Bleacher report:
Other Illegal Hits on Players in Compromised Positions
One of the dirtiest hits in recent memory came on a play that is now illegal. Defenseless players are now protected after a change in possession. Here is Hall of Fame defensive tackle Warren Sapp blowing up Packers offensive tackle Chad Clifton:
At the time, this was legal a football play. Sapp didn't know which direction Clifton would move and whether he might make the eventual tackle, so Sapp played all out until the whistle—just like young players are taught.
It's also clearly a dangerous play, which is now against the rules.
Players are also protected from blows to and from the head during blindside blocks in which an offensive player is moving parallel to the line of scrimmage, as well as illegal crack-back blocks in which the offensive player uses his head. Let's just call this one the "Hines Ward Rule." Here's Seattle Seahawks receiver Golden Tate being called for such an illegal block on Dallas Cowboys linebacker Sean Lee:
This is a Bleacher Report article and you can serach using "Warren Sapp hits now illegal"