The St. Louis Post-Dispatch reported Monday that Duncan was suffering from a sports hernia, which is defined as "a weakening of the muscles or tendons of the lower abdominal wall," though before the game, a club spokesman said no official diagnosis would be made until Duncan met with team physicians Monday afternoon. The spokesman referred to Duncan's injury as a vague "groin discomfort."
Cardinals trainer Barry Weinberg told the Post-Dispatch that Duncan would need surgery at some point, but they weren't sure if it was immediately necessary. Duncan will definitely miss the upcoming series at Cincinnati. The Cardinals could know his status by the time they leave Chicago.
What is known now is that Duncan has been playing through an injury for well over a month, something that was kept quiet until he felt a tweak in his groin area running the bases Saturday in Arizona. His numbers have reflected his pain. He's 2-for-21 in September after hitting just .191 with one homer and seven RBIs in August. Duncan is hitting .259 for the year.
Surgery would sideline Duncan for the rest of the season, but the team is holding out hope he can return for the stretch run, at least in an abbreviated role.
When asked if Duncan's return was "suspect," Cardinals manager Tony La Russa said, "Suspect sounds worse than questionable. So I'll call it questionable."
The Cardinals are no strangers to adversity this season, but Duncan's absence in the middle of the lineup could be the last straw.
"We seem to roll with [adversity]," second baseman Aaron Miles said, "but he's a big part of the team."
The slugging outfielder has been the Cardinals' second-best run producer this season with 21 home runs and 70 RBIs. The Cardinals came into Monday's makeup game with the Cubs in third place in the tight National League Central, three games behind Milwaukee and two behind the Cubs with 22 to play.
As of Monday morning, La Russa wanted to give the slugger the "benefit of the doubt" that he can come back. If Duncan can return, he can be a dangerous weapon off the bench. He's 7-for-25 with two homers and eight RBIs as a pinch-hitter this season, including a grand slam on June 16 at Oakland.
So Taguchi started in left field Monday against left-hander Ted Lilly, and Skip Schumaker and Ryan Ludwick should get some extra starts in Duncan's absence.
"It's tough," Miles said. "He's been contributing to us all year and fighting through an injury. The kid's been playing hurt for a long time this year, basically taking it for the team."
Filling the hole: With Duncan out and Rick Ankiel stationed in right, Ludwick should see more time in left, especially against right-handers. He's hitting .287 (37-for-129) with eight homers and 25 RBIs against righties this season. La Russa doesn't know why Ludwick is hitting righties so much better than left-handers (.211, 23-for-109) this season. Ludwick wasn't even aware there was that much of a difference.
"There's no good reason to explain it if you look at his approach," La Russa said. "His approach throughout his career -- he's hit left-handers. When he's been seeing the ball well, it's been [against] right-handers. He's gotten some hits against left-handers, but the average just doesn't reflect it."
Ludwick sees some opportunity this month.
"We're all together as a team and anytime you lose a player like Duncan, it's going to hurt," Ludwick said. "But maybe it's going to be a case for me to step in and get more at-bats and be productive for this team."
He'll likely get some starts at Cincinnati this week, facing three right-handed starters.
One and done: Both teams had to fly in Sunday for this makeup game. The Cubs came in from Pittsburgh and then head to Houston for a three-game set and St. Louis for four games in three days. The Cardinals head to Cincinnati before returning home for 11 games in 10 days.
La Russa was nonplussed over the issue, though Cubs manager Lou Piniella fretted over the loss of an off-day when the Cubs have two in five days at the end of the month.
"It's a big advantage for Milwaukee," Piniella said.
The Brewers were at Pittsburgh on Monday evening and are set to host Cincinnati this weekend as the Cubs and Cards square off.
Rotation in flux: The Cardinals still haven't announced a starter for Wednesday's game against the Reds. Mark Mulder (0-1, 13.50 ERA) makes his much-anticipated second start Tuesday and Adam Wainwright (13-10, 3.75) goes Thursday. Wednesday will likely be another "bullpen start," like Sunday's loss at Arizona.
As for the Cubs series, Braden Looper will go Friday and Joel Pineiro will pitch one of the doubleheader games Saturday. Mulder's performance Tuesday will determine when he goes again.
The Cubs are throwing right-hander Carlos Zambrano Friday, the left-handed combination of Lilly and Sean Marshall on Saturday and righty Jason Marquis on Sunday.
News and notes: The Double-A Springfield Cardinals are the only Minor League team still alive for the Cards. They start a best-of-five Texas League Championship Series on Tuesday against San Antonio. ... Jim Edmonds homered in the second inning Monday, his 17th all-time at Wrigley Field and 11th this season. He's hit more homers at Wrigley than any other road park. ... Albert Pujols was booed vociferously by a nearly capacity crowd that was a bit more partisan than normal thanks to the makeup game situation. Ankiel, up next, was booed worse. ... The game was a makeup of the Aug. 19 rainout.
Coming up: Mulder faces off against Matt Belisle (7-8, 5.44) at 6:10 p.m. CT on Tuesday. Mulder allowed six runs in four innings in an 8-2 loss to the Pirates last Wednesday in his first start of the year following left shoulder surgery.
Jon Greenberg is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.