The NFL Draft rumor mill has kicked into overdrive, and now the Philadelphia Eagles and San Diego Chargers are reportedly back in the mix to trade up for Marcus Mariota. While the ties between Mariota and Eagles coach Chip Kelly set the stage for an obvious fit, the Chargers are a little more awkward.

Part of the reason for the Mariota speculation regarding San Diego is growing friction between the front office and star quarterback Philip Rivers. He is against relocation to Los Angeles, and with that on the table he's refused to talk about a long-term extension with the Bolts. His contract expires after the season.

All of that is alarming, but it is still unlikely that Rivers gets moved ahead of the draft. Here are the three biggest reasons why.

Age

The Tennessee Titans own the No. 2 overall pick, and have said publicly they're comfortable going with Zach Mettenberger for the foreseeable future. That might seem insane with Rivers potentially on the table, but Rivers is 33 years old and will be commanding a superstar contract.

This is a veteran quarterback who has made 144 consecutive regular season starts and has six 4,000-plus yard seasons under his belt. There are no discounts for players like that, and he is a pocket passer who limits the opportunities for defenders to tee off on him. Mettenberger is far from Rivers' class right now, but he is 10 years younger and the Titans believe in him.

Is Marcus Mariota A Starter Right Away?

It's one thing to move away from a veteran quarterback in favor of a new hotshot-the Chargers did just that after dumping Drew Brees to move forward with Rivers. When they acquired Rivers, however, they were picking up a prototypically sized pocket-passer that was NFL-ready from the jump, even if he didn't play right away.

Mariota's stats from last year jump off the page. He threw 42 passing touchdowns and rushed for 15 more, while getting picked off just four times. He even caught a touchdown pass. He ran the fastest 40-yard dash at the NFL Combine among all QBs (4.52). Athletically, it's all there. But he did not play in a pro-style offense at Oregon (excluding Philadelphia) and there are questions about his ability to quickly master NFL game plans.

No. 2 overall is a big investment in a player who might not be ready. In some cases the talent is worth the risk, but San Diego risks a PR nightmare if they dump a Pro Bowl caliber vet whose been the face of the franchise for nine straight years for a raw rookie.

The Compensation

Rumor has it the Browns are willing to trade away two 2015 first-rounders, and even that may not be enough to convince Tennessee to move off the second pick. The Eagles have been crazier than the Joker in The Dark Knight, and who knows what they might do as we inch closer. The Chargers simply might not have enough to give Tennessee if they decide Rivers isn't in their plans; either that, or they'd have to give away a sizable amount of their future like the Redskins did when they dealt for Robert Griffin III. The pieces just aren't coming together.