Feret said that the club have struggled to cope with the injury to Romain Alessandrini and have yet to find the "right forumula".
"Jonathan Pitroipa has moved to the left but he doesn't have the same profile as Romain," Ligue1.com quotes Feret as saying. "We're trying to adapt and for now we've not come up with the right formula.
"We've had something of a breakdown in terms of being effective. We're not putting attacking moves together well enough. Whether it's defensively or in an attacking sense, we're just not doing what is required of us to keep clean sheets and score goals."
Rennes will also face Saint-Etienne in the final of the Coupe de la Ligue in April.