The left-back has made just 12 minutes for Barcelona this season, with Flick preferring Xavi Espart and, more recently, João Cancelo.
Balde had previously been Barcelona's first-choice left-back but saw Cancelo move ahead of him during the second half of last season.
The emergence of Espart has now pushed him further down the pecking order.
Flick has reportedly identified a decline in Balde's performances in both matches and training and wants the 22-year-old to compete with the other left-backs in the squad.
His current situation represents a significant change for a player who was a regular for Barcelona and Spain, including at the 2022 World Cup.
Balde has one year plus an additional season remaining on his contract and resisted interest from Manchester United during the summer.
His representative Jorge Mendes also attempted to ease concerns about the player's situation after meeting Barcelona president Joan Laporta.
“Balde will be fine in the future, don't worry,” Mendes said when asked about his client.
However, the lack of opportunities could force the issue when the January transfer window opens. United remain an option for the wing-back.
Barcelona's summer departures did not generate as much financial flexibility as the club had hoped, while Balde remained at Camp Nou despite United's interest.
The situation could therefore become increasingly difficult if Flick continues to favour Espart and Cancelo.
For now, Balde remains at Barcelona and will have the opportunity to compete for his place, but another prolonged spell on the bench could lead him to reconsider the decision to stay.