Rams' Dunbar handed four-game suspension

St. Louis Rams linebacker Jo-Lonn Dunbar has been suspended for the first four games of the upcoming 2013 season after testing positive for a performance-enhancing substance, NFL.com reported on Wednesday.

Dunbar is coming off the most productive season of his five-year NFL career, compiling 115 tackles and a personal-best 4 1/2 sacks along with two interceptions while starting all 16 games for the Rams at weakside linebacker. The 28-year-old signed with St. Louis as a free agent prior to the 2012 campaign after a four-year stint with the New Orleans Saints.

The former undrafted free agent will be eligible to return for the Rams' Week 5 matchup against Jacksonville on October 6. Dunbar can still participate in St. Louis' three remaining preseason contests as well as training-camp practices.

Veteran Will Witherspoon, whom the Rams signed just prior to the start of camp, will likely man Dunbar's position during the suspension.