The Colts announced Friday morning that they have placed CB Marlin Jackson on injured reserve, making him the second defensive starter to be lost for the season this week. The team signed veteran free-agent DE Josh Thomas to take his spot on the 53-man roster. 

Jackson, who had played in only four games this season after missing the past three games with a strained ACL in his right knee, suffered a left knee injury — reportedly a torn ACL and meniscus — during a non-contact drill early in Wednesday's practice. He is expected to have surgery next week.

The fifth-year veteran, whose contract expires at the end of the season, had made 23 consecutive starts at cornerback before tearing the ACL and meniscus in his right knee during a non-contact practice in October 2008. He had been replaced in the starting lineup at right cornerback by rookie Jerraud Powers, whose steady play has solidified a secondary that has been riddled by injuries this season. In addition to Jackson, both CB Kelvin Hayden and SS Bob Sanders have been out for extended periods with injuries.

In fact, the team announced that Sanders visited with Dr. James Andrews to gather additional information on the left elbow injury he suffered last Sunday against San Francisco.

Also, the Colts revealed that WR Anthony Gonzalez underwent arthroscopic right knee surgery Wednesday to clean out loose particles. Gonzalez suffered a recent setback in practice that led to yesterday's procedure.  A timetable for his return is not determined.

As for Thomas, he played with the club from 2004-08, appearing in 68 games, with 12 starts and six sacks. He was cut from the team in training camp.