In addition to Sanders, Colts cornerback Kelvin Hayden is expected to miss up to a month with a knee injury, Colts coach Jim Caldwell said Friday.
Also, Colts cornerback Marlin Jackson tore his anterior cruciate ligament in practice Wednesday and already has been placed on the season-ending injured reserve list. A team source said Jackson tore the ACL on the final play of practice and is expected to have surgery next week.
As if that weren't enough, Colts wide receiver Anthony Gonzalez underwent arthroscopic knee surgery Thursday that will sideline him two to three weeks, a team source said. Gonzalez, who has missed most of the season, suffered a recent setback in practice that led to surgery to clean out loose particles in his right knee.
But the most damaging blow may be the injury to Sanders, one of the team's best defensive players.
In Sunday's win over San Francisco, Sanders hurt what the Colts initially said was his elbow. Sanders spent Thursday meeting with noted surgeon Dr. James Andrews in Alabama, going over the injury and how much damage the Colts safety could do to it if he were to continue playing.
On Friday, the team placed Sanders on the injured reserve list, ending his season.
Sanders already had missed the first five games of this season after undergoing offseason knee surgery. He returned to play at St. Louis and against San Francisco before injuring the biceps tendon in the latest injury that will end his season.
Thomas played with the Colts from 2004 through 2008, starting in 12 of 68 appearances, and was credited with 172 tackles, including six sacks.
Santi appeared in six games with the club last season, and he has spent time this year on the practice squad. He had 10 career receptions for 64 yards and one touchdown.
Adam Schefter is an ESPN NFL Insider.