(AP Photo/Michael Conroy, File)

Chris Johnson, a three-time Pro Bowler, signed a two-year deal with the New York Jets.

“I have a fresh start. Now I am going to go out there with a chip on my shoulder,” Johnson told the Tennessean’s Jim Wyatt in a phone interview Wednesday. “I know a lot of people are doubting me. I want to prove everybody wrong who has doubts in me.”

The former ECU Pirate is one of only six players to rush for at least 1,000 yards in his first six seasons, along with Barry Sanders, Curtis Martin, LaDainian Tomlinson, Eric Dickerson and Corey Dillon. But Chris Johnson is out of the league in a year or two.

Over the last decade, five other players have made over 350 rushing attempts between the ages of 23 and 25 – four were out of the NFL by their 30th birthday:

Clinton Portis rushed 352 times for 1,516 yards in 2005 for the Washington Redskins as as a 24-year-old. His last NFL season was in 2010, when he rushed 54 times for 227 yards and two touchdowns over five games at age 29.

Cincinnati’s Rudi Johnson carried the ball 361 times in 2004 as a 25-year-old, earning him a trip to the Pro Bowl, then played his last NFL game in December of 2008.

A 24-year-old Jamal Lewis averaged over 129 yards a game in 2003 while carrying the ball 387 times for the Baltimore Ravens. His career would be over six years later at 30 years old.

Deuce McAllister, at age 25, started all 16 games for New Orleans and ran 351 times for over 1,600 yards, but would never start more than 14 in any of his final five seasons.

It took Adrian Peterson just one season in the NFL before leading the league in 2008 for yards (1,780) on 363 carries at age 23.  “Purple Jesus” turned 29 in March, making it quite possible he too could be at the end of his NFL career.

As you would expect with any running back who has endured a heavy workload, there are already concerns over Chris Johnson’s knee:

There were, we’re told, no alarms or red flags.  While either or both knees could keep him from spending another decade in the NFL, the NFL is a year-to-year proposition.  For the coming year, the knee is good enough for Johnson to play at a high level.  Eventually, it won’t be.