We'll find out over the next two months if he's still capable of being a workhorse.
The Miami Dolphins on Wednesday placed Brown on season-ending injured reserve with what's believed to be a broken foot. With very little backfield depth on their roster, the load will fall on Williams.
On Thursday night, Williams should make only his second NFL start since 2005 when the Dolphins play the Carolina Panthers.
Brown handled more of Miami's workload. He had 42 more carries than Williams and was one of the year's most productive goal-line backs. Brown had six goal-to-go touchdown runs. Only Adrian Peterson and Michael Turner have more.
When you think of Williams during his glory years, you recall a relentless warrior. Former Dolphins coach Dave Wannstedt would call Williams' number again and again and again. Williams owns two of the NFL's top 14 seasons for rushing attempts, carrying the ball 775 times in 2003 and 2004.
But Williams is 32 years old, a couple of years beyond the standard expiration date for running backs.
Through nine games, he has been extraordinary. But as players such as Edgerrin James, Shaun Alexander and Ahman Green can attest, a guy can smack the wall in a hurry when he's a thirtysomething feature back.
The only other running back with any carries on Miami's active roster is fullback Lousaka Polite, who is averaging 2.9 yards mostly on short-yardage attempts. Second-year pro Lex Hilliard has zero NFL attempts. The Dolphins recently signed undrafted rookie Kory Sheets.
But if Williams' performances to date indicates what he will do in Brown's place, then the 1998 Heisman Trophy winner will have a special season.
Williams was on pace to rush for 992 yards as the sidekick. If he stays healthy, that figure should go up. If he can reach 1,000 yards, he would break the NFL record for going six years between 1,000-yard seasons.
Williams has more yards from scrimmage than Brown on 38 fewer touches. Williams is averaging 6.1 yards for every rushing attempt or reception. Brown averaged 4.6 yards.