Here we are in Week 3 of the NFL preseason. Yup, it's go time!Teams use this week as the dre s rehearsal for the regular season. Of course, every team has things to iron out before the show opens. Major questions remain, especially when it comes to certain players.Here are nine guys carrying substantial question marks into the preseason's most important game:1) Steven Matz Jersey Peyton Manning, QB, Denver BroncosThat was , speaking to The MMQB's Peter King and explaining that he hasn't had feeling in his fingertips since the series of neck surgeries that caused him to mi s the entire 2011 season. Pretty jarring stuff. Now, he has thrown 131 touchdown pa ses in the three seasons since, but this speaks to a larger concern for Manning: the corroding health of his 39-year-old body.Let's get one thing straight: Manning is an all-time great quarterback. There's clearly no disputing this. But 17 seasons deep, his body is betraying him. He wasn't healthy down the stretch last season, as a quad injury drastically impacted his play -- and led to Denver's playoff one-and-done, via. Can we really expect Manning's body to hold up through another long NFL season? Oh, and did I mention that the ' offensive line is chock full o' inexperience? In Week 2 of the preseason, Denver started two rookies on the left side (tackle and guard ) and a center who spent last year on the practice squad (). Now, on Wednesday, , who will take over for Garcia at left guard. That's a major upgrade. But Manning still has a lot of feeling out to do with all of the new faces on his O-line.And just in case that wasn't a big enough list of concerns ... Peyton is learning a new offense, with Gary Kubiak taking over as head coach. And his first trial run in live action -- -- certainly left something to be desired. Manning completed eight of his 14 pa ses David Wright Jersey for just 52 yards (a paltry 3.7 yards per attempt). Most importantly, all four Manning-led drives in the game ended in punts. It appears there's going to be a learning curve for everyone to grasp Kubiak's offense, and for Peyton to get comfortable performing all of his checks at the line. It will be very interesting to see where everyone's at in .2) Devin McCourty, DB, New England Patriots for being an impactful and reliable safety. Yet, in , McCourty lined up at cornerback. And in the postgame, the ultra-cla sy and team-oriented 28-year-old didn't seem too keen on the move."I hope it's not permanent," . "As a player, that's out of your control."It didn't feel great, and I don't think it looked great, so we'll see."With gone to the and in New Orleans, Bill Belichick is taking a look at all of the different po sibilities in his secondary. McCourty , so maybe that was just a one-game trial. hero and appear poised to start as the top two cornerbacks, with a battle still raging for the nickel spot.The ' upside in 2015: to win another Lombardi Trophy. That said, with some ambiguity in the defensive backfield -- not to mention at quarterback -- there's notable uncertainty about the defending champs. might not be the most imposing test for the secondary, but it will still be interesting to see how Belichick deploys his troops.3) Brandon Scherff, OG, Washington RedskinsYes, 's poor play and complete lack of pocket awarene s is discouraging, but let's be honest: The ' offensive line was atrocious in . It was particularly alarming seeing the fifth overall pick resemble a fifth-round selection. Scherff got completely worked over at right guard. Just look at . Can't imagine that was fun for Scherff to relive in the film room.As I wrote in immediately following the first round of the 2015 , Scherff was a big-time reach at the No. 5 overall spot. (Still can't believe the 'Skins Darryl Strawberry Jersey took him with just dangling there for the picking.) Scherff needs to shore up his play. And fast. On Saturday, Scherff and Co. to face the ' stout defensive front. How will the rookie guard perform against the imposing interior duo of and ?4) Darren McFadden, RB, Dallas CowboysHe's been injured in the preseason, which can't be a surprise. After all, McFadden's logged 16 games in just one of his seven NFL seasons. He did play in , though, logging 4 yards on three carries. All in all, head coach Jason Garrett was not happy with the ground game after that contest."It was good to see those guys out there," Garrett said, , alluding to the fact that Dallas' backfield has been quite banged up, "but we obviously didn't run the ball real well."No real surprise, if you ask me. With reigning Offensive Player of the Year gone to Philly, the ' RB depth chart includes the likes of , , and McFadden. I never would've signed McFadden in the first place, given his extensive injury history. Is he in jeopardy of not making the team? It'd certainly be nice to see him do something ...5) Devin Funche s, WR, Carolina Panthers, and that's just a brutal lo s for Carolina. I liked Funche s as a second banana to Benjamin, but is he ready to take over as the team's WR1? I have my doubts, that's for sure. And it certainly doesn't help that Funche s has been limited by a hamstring injury (though he did return to practice in a limited Tim Tebow Jersey role on Tuesday).Hopefully, the second-round pick is able to play in , because -- fair or unfair -- he needs to prove he can be an instant-impact target for . There is a ton of pre sure on this kid in the wake of Benjamin's injury. He needs all the preseason reps he can get.6) Jarryd Hayne, RB/PR, San Francisco 49ersWhat a story this is, with the Australian rugby star making an immediate impact on the Niners, particularly on special teams. It's highly entertaining to watch -- check out . Right now, he's averaging 21.6 yards per punt return. Oh, and he's also rushed 13 times for 117 yards.But is Hayne a lock to make the team?I think he's a front-runner, but he's not quite there yet. A guy like Hayne doesn't have any margin for error and needs to keep proving he belongs. I hope it continues to happen . This is just a fun guy to have in the league.7) Davante Adams, WR, Green Bay PackersWith the crushing blow that is , this second-year pro has been thrust into a leading role in the ' pa sing game, starting alongside . Is Adams ready to deliver?I believe he is. Mike McCarthy gushed about Adams in recent months, . We saw him flash plenty of talent as a rookie -- see: -- and I think he catches 80 pa ses this year.I believe the are still a legit contender because is the best quarterback in the game today. But Nelson was a top-five receiver. Adams has big shoes to fill. And it begins .8) Doug Martin, RB, Tampa Bay BuccaneersBucs fans had to be very enthused by the way Martin carried the rock . With six rushes for 59 yards -- including -- it was a throwback to Martin's 1,454-yard rookie campaign in 2012. Now, it needs to be the norm. looked much better in Week 2 of the preseason, but Yoenis Cespedes Jersey the rookie signal caller certainly needs help from the run game if the are going to surprise folks this year. Let's see what Martin can do .9) Blair Walsh, K, Minnesota VikingsAs , the are my surprise pick for the playoffs. But the kicker has to make kicks.Walsh, who , has come down with a rather absurd case of the yips this August. , Walsh mi sed all three of his field-goal attempts (including kicks of just 35 and 38 yards) and shanked an extra point for good measure.That has to end, right? Imagine Mike Zimmer would like to see his kicker boot a few balls through the uprights .Follow Adam Schein on Twitter .