"He's interesting," Gruden said, with a smile, of Mariota. "He took off a couple times today and it really fired me up. He's been hurt, but looks like the ankle really turned a corner. He's a dazzling playmaker with his feet, and that's the key to his game.
"I saw glimpses of that today. It's exciting. Started off slow on 7-on-7 [drills], but [he] picked it up, had a nice day. Had a real nice day."
Indeed, Mariota, who lost his starting job with the Tennessee Titans to Ryan Tannehill last season, struggled early in practice, missing tight end Jason Witten badly on an intermediate pass to the right sideline. And he throws a different ball than Carr.
But it is Mariota's scrambling ability and willingness to extend plays with his legs that makes him a good fit for Gruden's offense. Even as Mariota, the 2014 Heisman Trophy winner, has said since signing as a free agent with Las Vegas in March that the Raiders were Carr's team.
In fact, both Mariota, the No. 2 overall pick of the 2015 NFL draft by the Titans, and Carr, a second-round pick of the Raiders in 2014, suffered season-ending broken legs on the same day in Week 16 of the 2016 season.
"It's like weird, crazy things that link you together," Carr, 29, said earlier in camp.
"I'll tell you one thing, in our quarterback group you have to compete, and that's what I do. Anyone that's around me, all I'm going to do is compete. I've had multiple starters in the NFL come in here and be in the same room as me. You can go through the list about who's started games and who's been in our quarterback room. It happens all the time, but when you go 7-9, people like to make up stuff."
Raiders offensive coordinator Greg Olson said Mariota, 26, would push Carr, a three-time Pro Bowler and the franchise's all-time leading passer who is coming off career highs in passing yardage (4,054), completion percentage (70.4%) and Total QBR (62.2) but is just 39-55 as a starter, with one winning season in six years.
And as Raiders owner Mark Davis told ESPN.com, "The best quarterbacks are the ones that have the wins. Stats will follow."
Mariota is 29-32 as a starter.
"Competition brings out the best in any player in any sport," Olson said. "I would say it's the best competition that we've had since we've been here."
Gruden fired up by Mariota's recent performance
Jon Gruden tells the media why he's been so impressed by the recent performance of Marcus Mariota at camp after making a return from injury.