Mulder had been scheduled to make a short appearance for Triple-A Memphis on Monday night, but he was scratched after coming down with some back stiffness. He will fly to Detroit to join his teammates on Wednesday. At that time, he will be re-evaluated, and a decision will be made in the coming days about Saturday's starter.
If Mulder does not pitch Saturday, rookie Mitchell Boggs will take the ball.
"We will get a personal view of what kind of shape his back is in, and make a decision based on that," La Russa said. "Rather than have him with the Triple-A team, and the Triple-A team evaluating him and explaining it, we'll see for ourselves and then try to do something intelligent there, as far as we can."
The rest of the club's rotation for this week appears set. Kyle Lohse will pitch Wednesday, and Todd Wellemeyer is scheduled to pitch Thursday's series finale in Detroit. Friday in Kansas City will be Joel Pineiro, and Braden Looper will pitch on Sunday against the Royals.
Wellemeyer has experienced no setbacks as he attempts to recuperate from some right elbow soreness. If he does not recover well from a light bullpen session on Tuesday, he could still be scratched. However, that possibility grows less and less likely, and Wellemeyer seems to be a go for the start.
Matthew Leach is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.