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This is simple. If you're still searching for a franchise quarterback and at least one other team believes you might have one on your roster, that should serve as an indication that said quarterback is virtually untradable.

The Philadelphia Eagles are still searching for a franchise quarterback, and according to NJ.com's Eliot Shorr-Parks, the St. Louis Rams "have interest" in 2014 Week 1 starter Nick Foles.

In fact, per the same report, the Rams might not be the only team expressing interest:

Foles' possible availability was a topic of conversation among coaches and scouts last week at the Senior Bowl, with the Tennessee Titans and Houston Texans also mentioned as teams that would have interest if Foles hit the market.

The Eagles have to be in the business of collecting quarterbacks, not selling them. So unless the Rams, Titans or Texans would be willing to part with first-round picks in exchange for Foles, Philly would be silly to sacrifice him at this stage.

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There was talk last week that the Eagles, who currently hold the 20th overall selection in the 2015 draft, might be tempted to move up the first-round board in order to reel in Heisman Trophy-winning Oregon quarterback Marcus Mariota, who is widely considered to be the best signal-caller in this year's class.

If trading Foles is the only way to make that happen, it's not out of the question. However, that should be the only way Philadelphia would be willing to part with Foles—in order to land another pivot whom the Eagles have more faith in.

There's no way in hell Tennessee would surrender the No. 2 overall pick for Foles, though, and the Rams and Texans pick 10th and 16th, respectively.

Interestingly, according to the draft trade value chart, the typical cost of moving up from the 20th spot to the 10th would be about a mid-second-round pick.

Based on the fact quarterbacks Kevin Kolb and Alex Smith were dealt for second-rounders in relatively recent trades, that's probably about where Foles' value lies.

The problem is there's a very good chance Mariota won't be available in that spot, anyway. In his latest mock draft, Bleacher Report's Matt Miller had Mariota going first overall. What's more, 58 percent of the mock drafts conducted in the last week and tracked by popular draft site Walter Football had Mariota going in the top five.

Philly can't deal a player like Foles in hopes of landing Mariota. It has to be a sure thing. That means this might be the type of trade that can only take place on draft night.

For now, it should remain far-fetched.

If we're viewing Foles as a stock, this is quite clearly the wrong time to sell.

In 2013, the 2012 third-round pick out of Arizona set a new NFL record with a touchdown-to-interception ratio of 27-to-2 while posting the third-highest passer rating in NFL history (119.2) and leading the league with a yards-per-attempt average of 9.1.

That was a small sample size, and with the surprise factor gone, he struggled with health and performance issues throughout the 2014 campaign.

Foles played in only eight games in his third season, finishing with a sub-60 completion percentage and an 81.4 passer rating, which ranked 27th among 33 qualified quarterbacks.

The question, now, is which season was an aberration?

Did Foles come back to earth in 2014 because he's only an average quarterback? He didn't have a lot of help early due to injuries along the offensive line and a lack of support from a shoddy running game and a receiving corps that lost its top weapon, DeSean Jackson, in the offseason.

Then again, running joke of a backup Mark Sanchez outplayed him in relief, and it should be noted that Foles generally struggled entering the NFL as an afterthought. Six quarterbacks were selected ahead of him for a reason.

Anyway, the jury's still out. The Eagles might have strong hunches, but nobody knows for sure whether Foles is cut out to become the type of quarterback who can help this franchise break out of a half-century-long championship drought.

"I don't know," head coach Chip Kelly said when asked after the season if he could commit to Foles for 2015, per Mark Eckel of NJ.com. "We'll sit down and thoroughly evaluate everything."

Because this is a league with 32 teams and fewer than 32 great quarterbacks, and because there's strong evidence from the last decade that you can't win in this league nowadays without a superb starter under center, the Eagles don't have the luxury of giving up on a guy who has shown glimpses of stardom.

Nobody's going to surrender a potential franchise quarterback in exchange for another one, so unless the Eagles can part with Foles while guaranteeing they'll wind up with a better pivot in the draft, they'd be best served holding onto a potential star.

Brad Gagnon has covered the NFC East for Bleacher Report since 2012.

Follow @Brad_Gagnon