"It's bothering me just in my back, sometimes like in a nerve, just going through my muscles like a little fatigue," Mujica said. "We've just been working on it a lot and trying to get it ready to go."
Mujica pitched 1 1/3 innings and earned his 33rd save on Wednesday, but he was straining enough to warrant a mound visit from the trainer. The Cardinals had given Mujica three days off over the weekend due to what was described at the time as arm fatigue. Mujica noted on Thursday that he has been dealing with this muscle issue since making a two-inning appearance against the Pirates one week ago.
That was the third straight two-inning appearance Mujica had made.
"I felt it after the game ... and when I got in to Chicago, I just started feeling a little tightness in my neck, and when I went Tuesday, I started feeling it in the back," Mujica said.
If, while Mujica is sidelined, a situation arises to where the Cards need a fill-in closer, those duties would likely fall to Trevor Rosenthal. Rosenthal pitched a scoreless ninth with a four-run lead in Thursday's 6-2 win.
Asked about the immediate outlook for Mujica, manager Mike Matheny said the club remains "hopeful for [Friday]."
Jenifer Langosch is a reporter for MLB.com. Read her blog, By Gosh, It's Langosch, and follow her on Twitter @LangoschMLB. This story was not subject to the approval of Major League Baseball or its clubs.