Thomas is on injured reserve. The league and the Colts did not say what prohibited substance he tested positive for.
Thomas was a seventh-round selection by the Colts in the 2009 draft and played in eight games with Indianapolis in 2010. He was placed on IR on Aug. 19 this year with a back injury.
Foster dislocated his right ankle in a gruesome scene during Monday night's loss at Tampa Bay.
Ijalana, the Colts' second-round pick in April's draft, tore the ACL in his left knee during the game. The Colts (0-4) are also expected to be without starting left tackle Anthony Castonzo (ankle) this week as they prepare to host Kansas City (1-3).
To fill the holes, the Colts waived linebacker Nate Triplett, signed offensive tackles Michael Toudouze and Quinn Ojinnaka, and promoted defensive tackle Ricardo Mathews from the practice squad to the active roster.
"Practice will go along this week and we'll see where we are. We do have a lot of possibilities and some of them are too endless to go through," coach Jim Caldwell said Wednesday. "It's just the way it is. You take a look at where you are and adjust accordingly."
With Peyton Manning still out because of a neck surgery, and Kerry Collins not yet cleared to play after sustaining concussion-like symptoms Sept. 25 against Pittsburgh, Curtis Painter is in line to make his second straight start. Collins was limited in practice Wednesday.
"Right now, Curtis is our starter and Kerry can't practice at this point," Caldwell said. "So I'm not sure there's any need to go into any long elaborate reasoning."
Information from The Associated Press was used in this report.