The Jaguars' much-improved secondary suffered a blow this week when starting LCB Rashean Mathis (knee) was placed on injured reserve. Fellow starter Derek Cox will take over at left corner, with undersized reserve William Middleton stepping into the starting lineup on the right side. Middleton was up and down while starting four games earlier this season when Cox was unavailable. Defensive coordinator Mel Tucker, who was calling more man coverage with his corners this season, might scale back the aggressiveness following the injury to Mathis, we hear. A situation to keep an eye on will be the performance of Cox, who is playing the best football of his career but appeared to have his confidence shaken early last season when asked to play on the left side. It is a tough break for Mathis, a free agent at the end of the season who was putting together a solid campaign. Jacksonville could end up re-signing Mathis at a discounted rate because of the injury.