A late equaliser by Borja Oubina gave the relegation strugglers an unexpected point against the league leaders, and Fernandez said the result gave the team a much needed lift.
He told a press conference: "Given the situation we are in and the quality of the opposition, playing well against Barcelona and getting a draw was very important.
"Every point we get helps us and strengthens our team spirit. But now we need to make that point count by beating Rayo."
The Argentinian hopes he and his team-mates can replicate the performance they produced against Tito Vilanova's side.
He continued: "Without getting ahead of ourselves, we have to play with the same temperament and attitude we showed against Barcelona, because from now on every game for us is like a final.
"When you are in the situation we are in you can play well or badly, but you cannot afford to compromise on effort."
Fernandez warned of Rayo's potential in attack and hinted that Celta may play with caution against Paco Jemez's side.
He said: "When Rayo attack they can get you into a real mess because they've got some very able players and they are very strong going forward. We don't want an end to end game."
Rayo will be without forward Raul Tamudo, who will be out of action for three weeks after tearing a hamstring, but they welcome back midfielder Roberto Trashorras and goalkeeper Ruben Martinez, who both missed the defeat to Malaga last weekend.
Celta will be weakened by the absence of top scorer Iago Aspas, who is still serving a four-match suspension after being sent off against Deportivo La Coruna for violent conduct.
Rayo centre-back Jordi Figueras believes the loss of the striker, who has scored 10 goals so far this season, is a huge boost for his team.
He said: "Iago Aspas is fundamental. Before he was suspended he was as important as half the Celta team due to his goals, his passes and his skill.
"He has been banned for four games and now he's not playing his absence is really being felt."
Figueras spent a year and a half at Celta before he joined Rayo and said his return to Balaidos would be "special". He also said his side had to match their opponents in effort and urgency.
He explained: "Celta will be playing with anxiety and we need to play with that too.
"In this division there are 20 teams but there are no easy games, every team makes things hard for you."