Cards head athletic trainer Barry Weinberg said Mulder reported some soreness in the front of his left shoulder in the third inning Sunday. In that inning, Mulder surrendered a three-run home run to Cubs outfielder Matt Murton and gave up an additional run before being pulled from the game by manager Tony La Russa.
In three outings this year, Mulder has given up 15 earned runs in 11 innings, and hasn't lasted more than four innings in a start.
Weinberg said the lefty was scheduled to visit with the Cardinals team physician Dr. George Paletta on Monday to determine if an MRI was needed.
"It's the chicken or the egg," Weinberg said. "We don't know whether [bad] mechanics are causing the discomfort or the discomfort is causing the mechanics. We'll take a look at him this week and see what happens."
La Russa said the ideal plan this season was to bring Mulder back and have him start as many games as he could in September, and then have the winter to get to 100 percent.
"It was important to get him back out there. But, if he has discomfort, that doesn't make sense to push that," La Russa said. "The big thing is the elevation of the ball. He's going out there without all his weapons. That's a big issue."
After his start Sunday, Mulder said he actually started to feel some fatigue in the second inning. Mulder didn't report to Weinberg that anything was wrong until after the third, and never said anything to La Russa.
"That's the case for being a hero," La Russa said. "With everything he's already gone through with that arm, and I'll tell him this, the minute he feels anything, he's got to say, 'Time.' When you compare the importance of his healthy arm to [giving up] some runs in a game, they don't compare."
In each of this three starts, Mulder has gotten through the first inning without giving up a run. He said Sunday that he attributes that to adrenaline and is able to throw a little harder and be more accurate."
Surgery for Duncan: Injured outfielder Chris Duncan will undergo surgery for his sports hernia Thursday. The surgery will be performed in St. Louis by Dr. Michael Brunt.
Duncan hasn't started a game since Sept. 8 in Arizona. Since then, Duncan has appeared as a pinch-hitter twice, and both times has struck out swinging. Duncan had 21 homers and 70 RBIs this season.
The surgery will likely end Duncan's regular season.
Eckstein update: Shortstop David Eckstein said his sore lower back continues to feel better, but still isn't 100 percent.
As a result, Eckstein was out of the lineup Monday for the fifth straight game. The shortstop said he visited with a chiropractor Monday for an extended amount of time and was pleased with the visit.
"[It's] feeling better every day," Eckstein said, before hoping in the batting cages for pregame batting practice. "I'm sick of watching. ... [I'm] ready to get back in there."'
New spot for Ankiel: Right fielder Rick Ankiel batted in the fifth spot Monday for the first time this season.
La Russa said Ankiel is used to hitting in an RBI position because he did it so much at Triple-A Memphis. Ankiel has struggled recently, going just 4-for-33 over his last 10 games, with two of the hits coming in Sunday's series finale against the Cubs.
"If he was still going like he was when he first came up, you probably wouldn't move him," La Russa said. "Now he's struggling, so why not move him?"
Barden still waiting: Of all the players that were called up from Triple-A Memphis when the rosters were expanded earlier this month, only utility infielder Brian Barden has yet to get a start.
Barden was claimed off waivers earlier in the season from the D-backs, and then assigned to Triple-A Memphis.
"You have [Eckstein] and whatever his situation is there," La Russa said. "[Barden] could get a start or two there. He's shown some pop in his bat and [Triple-A manager Chris Maloney] has talked about his defense. He's capable."
Coming up: The Cards and Phillies will continue their three-game series with a 7:10 p.m. CT start Tuesday. The Cards have yet to announce a starter for the matchup, while the Phillies will send Cole Hamels (14-5, 3.50 ERA) to the mound.
Daniel Berk is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.