Martin Mayhew, the Detroit Lions general manager, has a theory that the roster is never final. Even after final cuts after training camp he tweaks and tweaks throughout the season.

Less than three weeks from the NFL draft. The Lions roster, like every NFL roster, is still in flux.

For my money, here are the top four roster vacancies:

1. Defensive end to replace Kyle Vanden Bosch who was released. This is critical. Jason Jones was signed as a free agent to fill the left defensive end spot opened when Cliff Avril signed in Seattle. BYU defensive end Ziggy Ansah could be available with the fifth overall pick. Willie Young (in photo sacking Drew Brees) is on the roster but not sure he’s ready to start. I thought they would re-sign Lawrence Jackson — and they might — but it’s not done yet. Also Ronnell Lewis is an option, but not a likely one as a starter. He didn’t play on D-line at all as a rookie last season and, in fact, was inactive the final seven games because he wasn’t contributing enough on special teams.

2. Cornerback to play next to Chris Houston. If Dee Milliner is available with the fifth overall pick in the draft, this would seem like a good fit. He might not be available and perhaps Mayhew and I don’t think along the same lines. It’s possible that either Bill Bentley, Chris Greenwood or Jonte Green could earn this starting spot. All three were drafted last season.

3. Offensive tackle for starting role to replace either Jeff Backus, who retired, or Gosder Cherilus who signed with the Colts. Riley Reiff, the first-round pick last year will likely start at one tackle. I think it’s quite possible that Corey HIlliard and Jason Fox could battle for the other starting tackle role. If they’re not good enough, what then? Two of the top draft picks are offensive tackles Luke Joeckel and Eric Fisher, but both could be gone in top four picks. 

4. Outside linebacker to fill Justin Durant’s role from last two years. It’s possible this could go to Ashlee Palmer who has played in all 16 games the last three years for Detroit as a backup, with two starts last season. They also want to see what their young guys can do there. They drafted Travis Lewis and Tahir Whitehead last season. Neither got a chance at linebacker, playing mostly on special teams. 

Editor’s note: Originally had kicker as a big need, but the Lions have signed veteran kicker David Akers to replace Jason Hanson who announced his retirement on Thursday.

(Follow @PaulaPasche on Twitter. Order her new book,  “100 Things Lions Fans Should Know & Do Before They Die’’ here. It’s also available at bookstores.)