Calvin Johnson has told people that 2015 would be his final season in the NFL, sources told ESPN, which would leave a massive hole for the Detroit Lions at receiver.

Johnson has a cap number of just over $24 million for 2016. If he follows through on retiring, the Lions would get at least $11.1 million back in cap savings. That would open up more money for the Lions to pursue a high-profile free agent, both at receiver and other positions. Or Detroit could address the void at receiver through the draft; it has the No. 16 pick.

Replacing one of the greatest receivers in NFL history is not going to be easy, or accomplished by one player, but here are several options between free agency, the draft and the current roster that could be part of the mix.

If the Bears happen to let Alshon Jeffery walk away, perhaps the Lions could pursue the wide receiver. Tim Fuller/USA Today Sports

1. Alshon Jeffery, WR, Chicago Bears: This would seem like a long shot, especially since the Bears could use the franchise tag or sign him to an extension before he reaches free agency. But if he does reach the market, the Lions likely would at least pursue the 6-foot-3 receiver who is hitting his prime. He struggled with injuries in 2015, but in four seasons, he has 252 catches for 3,728 yards and 24 touchdowns. In nine games last season, he had 54 catches for 807 yards and four touchdowns. Jeffery would be the closest experienced replica to a deep, game-changing threat the Lions could get. It's just unlikely that he will be available.

2. Marvin Jones, WR, Cincinnati Bengals: He is an interesting case. He has the size at 6-2 and speed to be a bigger deep threat and could be available if the Bengals don't want to pay him, especially considering they have a clear-cut No. 1 receiver in A.J. Green. In three seasons (Jones missed 2014 due to an ankle injury), he has 134 catches for 1,729 yards and 15 touchdowns. He's sure-handed with just six drops in three seasons on 215 targets. With Jeffery unlikely to leave Chicago, Jones could be the top target in an otherwise weak free-agent receiver class.

3. Laquon Treadwell, WR, Ole Miss: It's doubtful Treadwell reaches No. 16, but if he does, the Lions should target him. The 6-3 receiver had 202 receptions for 2,393 yards and 21 touchdowns in three seasons with the Rebels. He suffered a nasty leg injury in 2014 -- a broken fibula and dislocated ankle -- but showed no signs of long-term damage during a 1,153-yard season in 2015. The Lions have a lot of issues on the offensive and defensive lines, but they also need a long-term replacement for Johnson. Treadwell could provide just that from a size and speed perspective.

4. Will Fuller, WR, Notre Dame: The 6-0 receiver should be available at No. 16 -- which is probably too high of a pick for this talent -- and might be the most complete pass-catcher in the draft. He has good speed, runs routes well and is elusive once he has the ball. His hands could be an issue, but he has put up back-to-back 1,000-yard seasons and is a disciplined receiver.

5. Eric Ebron, TE, Detroit Lions: He is going to assume a larger role in the Lions' offense no matter what, but if Johnson retires, Ebron will no longer be the clear-cut No. 3 or No. 4 option. He has the speed to be a middle-of-the-field deep threat and the size to be a mismatch. The Lions occasionally lined him up outside and could do more of that in 2016 if Johnson is no longer a coverage problem on the outside. This was always going to be an important season for Ebron's development, but the Lions would have to rely on him more if Johnson departs.

Seven other possibilities: Mohamed Sanu, WR, Cincinnati Bengals; Travis Benjamin, WR, Cleveland Browns; James Jones, WR, Green Bay Packers; Jermaine Kearse, WR, Seattle Seahawks; Tyler Boyd, WR, Pittsburgh Panthers; Leonte Carroo, WR, Rutgers; and Aaron Burbridge, WR, Michigan State.