Former Washington Redskins head coach Mike Shanahan believes that Robert Griffin III's slump in form has nothing to do with his injury problems.

Griffin had an excellent rookie season in the NFL in the 2012 campaign, but then sustained a devastating knee injury during the playoffs.

The 25-year-old was rushed back for the start of the 2013 season, but struggled to find his form before being rested for the final three games of the campaign.

Griffin's problems continued last season when he was even benched for a period under new head coach Jay Gruden, leading to doubt over his role for the 2015 campaign.

"I don't think getting hurt has anything to do with it," Shanahan told WJFK. "I think what you do is you rehab yourself and you get better.

"You don't have to have great running ability to run the read-option. You have to be able to know when to slide, when to throw the football away, depending on if you're running or passing."

The Redskins have used the fifth-year option on Griffin's contract to extend his deal until the end of the 2016 season.