When running back Michael Turner became a free agent last off-season, there was no decision for the San Diego Chargers to make. Turner was expendable. The Chargers already had LaDainian Tomlinson, the best running back in the NFL. No sense overpaying to keep his backup.

The decision won't be as cut-and-dried this off-season with Darren Sproles, 25, set to enter free agency.

Turner signed with the Atlanta Falcons for $34 million and outplayed Tomlinson in 2008, finishing second in the NFL in rushing with 1,699 yards — almost 600 more than Tomlinson.

It was the second consecutive season Tomlinson's statistics dipped dramatically. He rushed for 1,815 yards in 2006 on his way to NFL MVP honors, then 1,474 yards in 2007 on his way to the Pro Bowl. But he won't be going anywhere after rushing for 1,110 yards in 2008.

Tomlinson played through injuries all season. In the first half of the year, it was a toe. Lately, it's been a strained groin. That injury limited him to five carries in San Diego's playoff opener against Indianapolis, and it probably will keep him out of Sunday's AFC divisional playoff at Pittsburgh.

But there's no reason for alarm in San Diego — not with Sproles coming off a career performance while filling in for Tomlinson against Indianapolis. Sproles rushed for 105 yards on 22 carries, culminating his night with a 22-yard touchdown run in overtime to sink the Colts.

Despite limited carries as the No. 3 back in the rotation behind Tomlinson and Turner in 2007, Sproles posted his first career 100-yard game (122) against Detroit.

A workhorse in college at Kansas State, Sproles converted 815 career carries into a school-record 4,979 rushing yards. He collected five 200-yard rushing games and 19 more 100-yard games, averaging 6.1 yards per carry.

Sproles was big enough to play in the Big 12, but apparently not big enough to play in the NFL. His lack of height (5-6) sank his draft stock into the fourth round, where the Chargers grabbed him.

Until last weekend, Sproles was viewed as an elite kick returner and a spot back. The Chargers may have to view him differently this off-season.

There are two realities about NFL running backs — your vulnerability to injury increases as you age and your productivity decreases.

Tomlinson turns 30 next season. That's ancient by running back standards. Only 24 backs in NFL history managed to rush for 1,000 yards once they reached their 30s.

It's a young man's game, and Tomlinson is no longer a young man. The salary cap won't allow teams to pay age and continue to compete.

So expect the Chargers to spend more time discussing Sproles' future this off-season than they did Turner's. San Diego must start preparing for life after Tomlinson.

Great players don't stay great forever. If the Chargers decide not to keep Sproles, look for them to spend a high draft pick on a running back in April.