San Francisco 49ers coach Jim Harbaugh isn't a fan of the NFL's interpretation of what a legal hit is on a quarterback when running a read-option play.  In light of the interpretation, defenders such as Green Bay Packers linebacker Clay Matthews are already planning on taking full advantage.

NFL vice president of officiating Dean Blandino described in a weekly rules video what an acceptable hit is on a quarterback running a read-option play, ESPN reports. 

"The quarterback can be hit like a runner until he's clearly out of the play," Blandino said, via ESPN.

The rule, in a deviation from the NFL's readiness to protect quarterbacks, arguably leaves the signal caller more vulnerable to injury when in the read-option offense.  Harbaugh expressed his contempt for the interpretation.

"I believe that when a quarterback is handing a ball off or faking a ball - in the read-option case, he's reading on an option play and he's as defenseless as a quarterback who's in the act of throwing," Harbaugh told reporters on Friday, via ESPN.

The league's interpretation is a boon for defenses.  In an era when rule are increasingly handicapping defenders, defenses can now try to deter the NFL's latest trend by putting helmets on quarterbacks who run read-option plays.

Defenses are already adjusting their gameplans.  Kaepernick faces the Green Bay Packers on Sunday, and Packers linebacker Clay Matthews said on Wednesday during "Mike and Mike" that his team is planning on taking advantage of the interpretation.

"One of the things that referees have told us is that when these quarterbacks carry out the fakes, they lose their right as a quarterback, a pocket-passing quarterback, the protection of a quarterback," Matthews said, via ESPN.  "So with that, you do have to take your shots on the quarterback, and obviously they're too important to their offense.  If that means they pull them out of that type of offense and make them run a traditional, drop-back, pocket-style offense, I think that's exactly what we're going for.

"So you want to put hits as early and often on the quarterback and make them uncomfortable."

Franchise quarterbacks such as Kaepernick, Robert Griffin III and Russell Wilson are suddenly much more prone to hard hits when they run the read-option play, putting them at greater risk for injury. 

The rule will either deter the prevalence of read-option plays or be changed by the competition committee when teams start losing their star quarterbacks for the season.