The Cleveland Browns picked the former Heisman Trophy winner, who has been taking some first-team reps during organised team activities this week, in the first round of this year's Draft.
The AFC North side's offensive coordinator told USA Today: "Johnny and [Robert Griffin III] are very similar. They're both talented guys who can make plays with their legs. But there comes a time in the NFL when they're going to keep you in the pocket, and you're going to have to have that ability to make the throws with your arm and mind.
"And Johnny, just like Robert, has that arm talent and intelligence to do that. Perception becomes everyone's reality, but Johnny has been here working his tail off day and night, doing everything we ask.
"He's really attacked it every day, done a good job even as each week gets harder as we throw a lot more at him. I've been very impressed."
Shanahan worked with Griffin at the Washington Redskins during his first two seasons in the NFL before moving to the Browns at the end of the 2013 campaign.