As we near the 2021 NFL season-opener, the pool of free agents is looking pretty bare. Most teams signed their big-ticket free agents earlier in the offseason and others have picked the pool clean over the course of training camp and the preseason. Some of the top free agents available are all at least 30 years old, but that doesn't mean they carry no value. Here are the top five available NFL free agents that are underrated and can easily help a team ahead of the 2021 NFL season.#5 - EDGE Olivier VernonOlivier Vernon is a free agent. He's still a solid player https://t.co/oIr3Xa16HY— Sosa Kremenjas (@QBsMVP) August 22, 2021In 2020 with the Cleveland Browns, Olivier Vernon played opposite Myles Garrett. He had nine sacks, 12 tackles for loss, and 16 QB hits. He tore his Achilles in Week 17 and is still a free agent. It's unclear what the 30-year-old defender's status is in his rehab, but he's a gem that several teams will jump on whenever he announces his return to the NFL.#4 - S Tre BostonTampa Bay Buccaneers v Carolina PanthersVeteran safety Tre Boston was a cap casualty by the Carolina Panthers as Jeremy Chinn emerged as a better player last year. He has started the last 32 games for Carolina with four INTs, 15 passes defended, one sack, 163 tackles, four tackles for loss, and three QB hits. He's only 29 and could still be a starter in the NFL. Is his price tag keeping him on the market? His contract with Carolina was just $6 million annually and that is quite affordable for his talent. The Panthers could bring him back on a discount, but a team like the New York Jets could benefit from his services after recent injuries on the roster.#3 - C Austin ReiterFormer #Chiefs C Austin Reiter still being a FA is surprising:*28 starts the last 2 seasons*No sacks allowed in 2020*77.0+ pass-blocking @PFF grade each of the past 3 seasons*1.6% pressure percentage allowed since 2018, second-best behind Rodney Hudson among all interior OL. pic.twitter.com/M3575caQK3— Ari Meirov (@MySportsUpdate) August 15, 2021Austin Reiter spent the last three seasons blocking for one of the NFL's best QBs in Patrick Mahomes and went to two straight Super Bowls with the Kansas City Chiefs. He's a balanced blocker with experience and fits in an outside zone scheme. Reiter has 32 starts over three seasons and allowed three sacks in that span. The Seattle Seahawks could land a championship-caliber center on their depleted OL with Reiter.#2 - OT Mitchell SchwartzKansas City Chiefs v Oakland RaidersKansas City also cut their All-Pro tackle Mitchell Schwartz after he suffered a back injury and played in just six games in 2020. Schwartz is 32 and his age/injury combo could be why he's still available. However, Schwartz has been uploading workout videos to his Twitter account that show fluid motion in his back and that his rehab is going smoothly. His price in 2020 was $8 million annually and a team could bring him in if a starter gets injured this season. #1 - LB Avery WilliamsonNew York Jets v Buffalo BillsAvery Williamson is only 29 with 15.5 sacks, 607 tackles, and 29 tackles for loss in six NFL seasons. He was traded to the Pittsburgh Steelers in the middle of last season but struggled to learn the defense on the fly. He played in eight games with four starts for 52 tackles and a sack. Williamson is a big-bodied ILB who thrives against the run. Pittsburgh has gone with bigger NFL free agents for their LB position, but Williamson can certainly still produce.