"Take it from a guy who had a real tough time leaving there -- I think it's a good decision," Kelly told Philly.com.
And while Pac-12 and national championships have eluded Mariota so far, the declared goal of his return is to earn his degree and spend another season with his friends and teammates.
Kelly pointed to the value Mariota's family places on education as a key factor in how it has shaped him as a football player.
"He's a really special young man," Kelly said. "I think everyone in the league will have to wait a couple of years. I love coaching him. He's exactly what you want in a football player."
With the Eagles at 7-5, tied for the lead in the NFC East, Kelly's first season in the NFL is more likely to end with a division championship than Oregon, which finished its regular season at 10-2 and second behind Stanford in the Pac-12 North.
Of course, Mariota's return could give Kelly the opportunity to reunite the offensive mastermind and quarterback through the 2015 NFL Draft. As Mariota was regarded at one point as the potential No. 1 overall pick in the 2014 draft, it would take a sophomore slump by Kelly's Eagles to be in position to do so.
If Kelly's second season with the Eagles mimics Mariota's second go-round with the Ducks -- accounting for 3,412 passing yards and 30 touchdowns against four interceptions, with 582 rushing yards and nine touchdowns this season -- pairing up again seems unlikely to happen.