Sweeney caught the ball on the warning track, and then pulled up limping. He flipped the ball to left fielder Ryan Kalish to make the throw back to the infield. The Cardinals had Matt Holliday at first and no outs at the time, but Holliday did not advance on the play. The Cubs held on for a 6-5 win.
Cubs athletic trainer Ed Halbur went out to Sweeney, who walked off the field under his own power.
"He had great determination and he got a great jump on it and got to it and kept his composure," Cubs manager Rick Renteria said of Sweeney.
The Cubs already have been playing short-handed in the outfield since Justin Ruggiano went on the disabled list on April 24 with a left hamstring strain. Chicago opted to carry 13 pitchers and take advantage of Emilio Bonifacio's versatility, rather than add an outfielder at that time.
However, the Cubs were expected to call up an outfielder from Triple-A Iowa, and could add Chris Coghlan, who was the National League Rookie of the Year in 2009 with the Marlins. Coghlan was batting .243 in 24 games with Iowa.
The other options at Iowa included Josh Vitters, who was batting .202, and Matt Szczur, who was hitting .267.
Carrie Muskat is a reporter for MLB.com. She writes a blog, Muskat Ramblings, and you can follow her on Twitter @CarrieMuskat. This story was not subject to the approval of Major League Baseball or its clubs.