The Chargers just locked up Keenan Allen to a four-year extension worth a reported $45 million, making him one of the 10 highest-paid wideouts in the league.

The move makes sense because Allen is one of the NFL's best young pass catchers. With Allen taken care of, who are the other wide receivers who are in line for new deals or, at the very least, grossly underpaid?

Julian Edelman, Patriots

Julian Edelman gives the Patriots great bang for their buck. USATSI

Edelman's 2016 cap hit is just $4.48 million, which ranks 33rd among all wide receivers. Meanwhile, in an injury-shortened 2015 season, Edelman had 61 catches for 692 yards and seven touchdowns in nine games. Extrapolated over an entire season, that works out to 108 catches, 1,230 yards and 12 TDs.

As former agent Joel Corry wrote for back in December, the Patriots played hardball with Edelman after the 2013 season, when there was little interest in his services on the free-agent market. He returned to New England on a deal that paid him $17 million over four years, including $8 million guaranteed.

For some perspective, Edelman ranked 28th in total value among all wideouts, according to Football Outsiders' metrics (total value is a cumulative statistic, by the way), which was just four spots behind Keenan Allen.

Unfortunately, don't expect the Pats to rip up Edelman's deal and give him a big pay bump; he just turned 30, which means that his best days could be behind him. Remember, New England moved on from a 31-year-old Wes Welker following the 2012 season, and that was after Welker had 118 receptions for 1,354 yards and six scores.

But who knows, with the expectation that Tom Brady could play well into his 40s, he'll need reliable targets, and no one fits that description better in recent years than Edelman.

Doug Baldwin, Seahawks

Doug Baldwin's best season came in 2015. USASTI

Baldwin's 2016 cap hit is $6.35 million, which ranks 25th among all wide receivers. Baldwin, a former undrafted free agent out of Stanford, is in the final year of a three-year, $13 million deal he signed before the 2014 season. There's a case that he's one of the league's most underrated players, particularly since he plays in an offense that is known more for it's ability to run the ball than its downfield passing game.

(Fun fact: Seattle's offense ranked No. 1 last season; No. 3 in rushing and No. 2 in passing. So yeah, so much for perceptions.)

Baldwin caught 78 passes for 1,069 yards and 14 TDs in '16, good for second among all wideouts behind only Antonio Brown, according to FO's metrics. And while Baldwin has been outspoken about what he and his teammates have accomplished, he's taking a decidedly low-key approach to a new contract.

"Again I'm just thankful for the opportunities that I have to be out here every day and I'm going to focus on that each day as it comes and just enjoying the time that I have with my teammates," Baldwin said this week, via the Seattle Times. ... "I'm playing football each day as it comes by. And again, if I focus on the task at hand all that other stuff will take care of itself. So I don't really have to think about it too much."

Antonio Brown, Steelers

The league's best receiver is nowhere near the league's highest-paid receiver. USATSI

Brown's 2016 cap hit is $12.4 million, which ranks eighth among all wide receivers. On the surface, it hardly seems like he's underpaid, but Brown has been the NFL's top pass catcher for two years running, via FO's metrics, and has been a top-five talent since 2013. Last season was Brown's best; he caught 136 passes for 1,834 yards and 10 touchdowns. And while it certainly helps that Brown has a franchise quarterback in Ben Roethlisberger, he's also undersized by NFL standards -- 5-foot-10, 180 pounds -- and was just a sixth-round pick out of Central Michigan back in 2010.

Of course, Brown might be elsewhere had Mike Wallace accepted the Steelers' $50 million contract offer prior to the 2012 season. Instead, as Wallace stalled, Pittsburgh signed the then-unproven Brown to a five-year, $41.9 million deal that was a risk at the time but is now one of the NFL's best bargains.

Despite the disparity between pay and production, Brown remains one of the hardest working players in the NFL.

"He works harder than anyone,'' backup quarterback Bruce Gradkowski said last month, per the Pittsburgh Post-Gazette. "That's something special. He does it on the field, but you should see what he does off the field - lifting, running, practicing. He does it all. It's cool to watch and guys get excited having him back."

And the Post-Gazette's Ed Bouchette writes that, "There has been no hint of any squabble over his contract that has two years left. Last season, the issue was raised, and the Steelers moved $2 million from his 2016 salary into 2015. So far, not a peep of discontent from Brown or agent Drew Rosenhaus about getting anything done this time around."

John Brown, Cardinals

John Brown's game is much bigger than his size. USATSI

Brown's 2016 cap hit is a mind-boggling $747,725, which is a function of the rookie deal he signed in 2014 as a third-round pick out of Pittsburg St. In fact, his four-year, $2.8 million contract, coupled with a breakout rookie season, means a hefty payday is in Brown's future.

Specifically, Brown caught 65 passes for 1,003 yards and seven TDs last season, and while those numbers don't jump off the page in this day and age of offensive football, Brown's 2015 output ranked fifth in total value among all wideouts, according to FO, behind only Antonio Brown, Doug Baldwin, A.J. Green and teammate Larry Fitzgerald.

Allen Robinson, Jaguars

Allen Robinson has quickly emerged as one of the game's top receivers. USATSI

Robinson's 2016 cap hit is $957,510, which again is due to the fact that the former second-round pick is in the third year of his rookie contract. And much like John Brown, Robinson's four-year, $3.5 million deal is a huge bargain for a player who hauled in 80 passes for 1,400 yards and 14 TDs last season, in a mostly under-the-radar effort in Jacksonville.

Robinson's numbers were good for eighth overall in total value, according to FO's metrics, which was better than other young pass catchers Sammy Watkins, Odell Beckham and DeAndre Hopkins. The good news for Robinson is that if he continues to be this productive, the Jags will almost certainly pay handsomely to keep him; the team has plenty of cap space, they have their franchise quarterback in Blake Bortles, and they just gave another young underrated talent, Allen Hurns, $40 million.

Honorable mention: Emmanuel Sanders, DeAndre Hopkins, Jarvis Landry, Willie Snead.