Manager Tony La Russa used Pujols, who's been out of the lineup since June 11 with a strained calf, as the designated hitter Thursday against Detroit to help ease his return to the lineup.
"We felt it was better for him to be tested with the at-bats before he does it defensively," said La Russa, who added that the six-time All-Star selection could return to first if he shows no ill effects.
Troy Glaus made his third start of the season at first base.
Pujols was hitting .346 with 16 homers and 42 RBIs before the injury. Driving in 100 runs this season will make him just the third player to begin his career with eight straight 100-RBI seasons. Ted Williams began his career with eight such seasons and Al Simmons is the all-time leader with 11.
Flores (1-0), who pitched an inning of Wednesday's 8-7 loss to the Tigers, has walked six of the last 14 batters he's faced. LaRussa said the club hopes the left-hander can begin a rehab assignment early next week and return to the team in two weeks.
His departure leaves the Cardinals with just one lefty -- Ron Villone -- on the roster.