The back-up quarterback, perhaps one of the most important positions on an NFL team's roster, gives the franchise an insurance policy should their starting quarterback go down with an injury.When Dak Prescott injured his ankle last season, the Cowboys had a decent back-up in Andy Dalton to fill in. However, many other teams do not have an experienced back-up and this can sometimes be their downfall.Many back-ups hardly see any time on the field at all and this is what makes having an experienced one behind your starter crucial.Here are the top five back-up quarterbacks in 2021Top five back-up quaterbacks in the NFL in 20211. Trey Lance (49ers)There was a reason why Kyle Shanahan and company drafted Lance. While still quite raw as a passer, his size and his ability to weave in and out of traffic is something starting quarterback Jimmy Garoppolo doesn’t have.SportsCenter@SportsCenterTREY LANCE WITH HIS FIRST NFL RUSHING TD RIGHT BEFORE HALFTIME 🔥(via @NFL)11:24 AM · Sep 27, 20212900245TREY LANCE WITH HIS FIRST NFL RUSHING TD RIGHT BEFORE HALFTIME 🔥(via @NFL)https://t.co/POrztC6eM0Having been used sparingly so far this season, should Jimmy go down as he has before, the 49ers are in good hands with Trey at the helm.2. Justin Fields (Bears)Like Trey Lance, Fields might rely more on his running ability than passing, but he has the physical tools to make the best of it in the NFL right now.NFL on ESPN@ESPNNFLJustin Fields getting ready for his first NFL start 🐻(via @NFL)12:53 PM · Sep 27, 202196198Justin Fields getting ready for his first NFL start 🐻(via @NFL) https://t.co/9nKl6nZDQ0With Andy Dalton injured, Fields was thrust into the starting job on Sunday against the Cleveland Browns, and he had a tough day. However, it is a vital experience for him and it would not be a shock if Dalton doesn’t get his starting job back.3. Case Keenum (Browns)Case Keenum is one of the most underrated back-ups in the league, and this was the case even when he was starting. He has experience and can extend plays, which he was known for with the Minnesota Vikings. Should Baker Mayfield succumb to an injury, then the Browns are in very capable hands.Sunday Night Football on NBC@SNFonNBCOn this day in 2018, Case Keenum and Stefon Diggs pulled off the Minneapolis Miracle(🎥 @NFL)2:37 AM · Jan 15, 20203102621On this day in 2018, Case Keenum and Stefon Diggs pulled off the Minneapolis Miracle(🎥 @NFL)https://t.co/teaBczR1H5Having playoff experience with the Vikings, Keenum has all the tools to be a starting quarterback in the league and the Browns are well-covered should the unthinkable happen.4. Jacoby Brissett (Dolphins)Brissett has experience in the NFL as a starter with the Patriots and the Indianapolis Colts and is now a back-up to Tua Tagovailoa. While not the most accurate passer, Brissett typically does a good job of avoiding turnovers during a game.SportsCenter@SportsCenterJACOBY BRISSETT SCRAMBLES FOR THE TD.The Dolphins got the two-point conversion and tied the game 😨(via @NFL)8:45 AM · Sep 27, 20211974161JACOBY BRISSETT SCRAMBLES FOR THE TD.The Dolphins got the two-point conversion and tied the game 😨(via @NFL)https://t.co/x4EKLxO6DSWith Tua injured for the next three weeks at least, Brissett started in the overtime loss to the Raiders on Sunday and looked the part. Brissett led the Dolphins on a game-tying touchdown drive and then a two-point conversion for good measure. The Dolphins are in decent hands with Brissett.5. Taysom Hill (Saints)Regarded by many as not the most polished quarterback and certainly not the best passer, Hill’s running ability causes problems for opposing teams. Yes, he does limit the type of play you can call, but when you are as gifted a runner as he is, it shouldn't be considered a negative.Cougs in the Pros 🤙🏼@BYUprosTaysom Hill powers it in from the 4 for his first touchdown of the year!! #BYUFootball | #Saints 5:26 AM · Sep 27, 202112410Taysom Hill powers it in from the 4 for his first touchdown of the year!! #BYUFootball | #Saints https://t.co/gKFnVOCMDEHill started four times last year when Drew Brees was injured and had a record of 3-1 and completed 72% of his throws, which is decent to say the least. With these five players, their teams are well-insured in case of sudden injuries to their starting quarterbacks.