The Romans sit fourth in the Serie A table and hold the final berth to qualify for Europe's top club competition.
With fifth-placed Udinese, who are two points behind them, showing no signs of slowing down, Muslera knows a victory is essential if they are to remain in the top four.
"At this point we are at a crossroads and if we can maintain our form we can do well," said Muslera.
"We have to try to remain in this position until the end."
Edy Reja's men go into the encounter inspired by last weekend's 2-0 triumph at Brescia but Muslera is aware of the threat posed by Sunday's struggling opponents, who are bottom of Serie A.
"It's going to be a very hard game," he said. "They will come here ready to take three crucial points in their battle to avoid relegation.
"Last year we lost against them at home and now we want to make amends."
The Galletti lie at the bottom of the table with just 15 points from 25 games and go into the game at the Stadio Olimpico nine points adrift from safety.
The arrival of coach Bortolo Mutti, who replaced the sacked Giampiero Ventura last week, inspired Bari to halt a run of six consecutive defeats with a goalless draw against Genoa last time out.
"It is still possible for us to avoid the drop," said striker Jose Ignacio Castillo.
"The situation is not easy, but we will take things one step at a time. We are ready for Lazio."
Bari have won just three times this season, their last coming against Lecce on January 6.
The hosts will be without injured pair Stefano Mauri and Stefan Lichtsteiner.
