The Nationals called up lefty Xavier Cedeno, giving them a fresh arm after Blake Treinen carried a heavy workload.
Ian Desmond has bounced back from vicious defensive slumps before, and he must try to again.
The issue has come up twice already for the Nationals but it doesn't mean players are happy with the new interpretation.
Denard Span expects to come off the seven-day DL on Saturday after a successful rehab game.
After an early slump, Harper is 14 for 29 with four walks, five extra-base hits and six strikeouts, and has an eight-game hitting streak.
The Nationals will try to beat the Cardinals for the first time since Game 4 of the 2012 NLDS.
If all goes well, Doug Fister could make his Nationals debut in the first week of May.
Bryce Harper returns to the Nationals' lineup against the Cardinals.
The outfielder hopes to return from his concussion on Saturday.
The Nationals went 0-6 against the Cardinals last season, and are 6-11 since the start of the 2012 season including the playoffs.