"We made mistakes at Rennes and we must learn from them," the Brazilian said.
The champions have seen their nine-point lead in Ligue 1 cut to three over second-place Montpellier since play resumed last month after the winter break and have taken only one win from their last four league outings.
They badly missed injured captain Alou Diarra in midfield against Rennes and lost 4-2.
"We were not used to dealing with such a large lead. But the league title remains our number one priority ahead of the Champions League", Fernando said. "Every time we are behind on the scoreboard we try to play as fast as we can."
The Brazilian, who recently complained about spending too much time on the bench, could fill in for Diarra, who has a groin injury, against St Etienne.
The Girondins, who already have one eye on their visit to Olympiakos in the Champions League on February 23, will seek revenge over St Etienne who were responsible for their first league defeat of the season in October.
St Etienne, 16th in the table, had a 3-0 victory over Monaco last weekend, a performance which put them seven points clear of the relegation zone.
In-form Montpellier, on a four-match winning streak, host bottom club Grenoble on Saturday while Olympique Marseille face a tough challenge at rivals Monaco.
Fourth-placed Olympique Lyon, who have yet to lose a league match this year, face Racing Lens at home.
