The announcement came amid concerns about the packed calendar as his side strive for titles in three competitions, Reuters reported.
Barcelona are on a 14-match unbeaten streak across LaLiga, the Copa del Rey and the Champions League and are in the running for titles in all three tournaments.
"It s not easy to manage and adjust. We ll do what we can. Having fresh players is what we ll do," Flick, whose side have four matches over the next 10 days, told reporters on Saturday.
"We have three competitions and we re fighting for three titles. When we started, nobody expected to be like this on March 1.
"There are a lot of things going on this season and I ve changed the training sessions," Flick said.
"In other games, like the (Copa del Rey) one against Valencia, we hardly rested and the team did great. We have to be focused on the game. There are no excuses... the calendar doesn t give us many more options. It s something that has an impact."
Despite conceding 25 goals this season, including four in their 4-4 draw with Atletico Madrid in the first leg of their Copa del Rey semi-final on Tuesday, Flick remains unfazed.
"I m not worried about when we concede... we start from scratch. What we have to do is take the energy and decide how to play," the 60-year-old said.
"We already saw the two goals against Atletico but then we played a good game. We invested a lot of energy at the end of the game too. We have to do things better as a team, we know that."
Barcelona will be without defender Andreas Christensen on Sunday as the Dane has been injured again.
Christensen, 28, missed the start of the season due to an Achilles tendon problem and then injured his right calf in training in January.
"Andreas has been injured and we have to wait and see how serious it is. Let s hope it s not, he has retired early. We have very good defenders.
"Ronald (Araujo) came in after an injury and is doing very well. He has everything to reach very high levels."
Barcelona top the LaLiga table with 54 points, the same number as second-placed Real Madrid, while Sociedad are ninth with 34 points.
