The Viola slipped to a 2-1 defeat at the Stadio Friuli on Tuesday evening, but Montella is determined that his players can turn the game around on home soil.
"It is absolutely all still open and we knew this was going to be tough against a balanced Udinese side who have quality and know how to cause you problems. Qualification was never going to be decided in the first leg," he told Rai Sport.
"There was great team spirit and it wasn't easy, especially with the rain, but the players dived into it. Of course it is annoying to lose, but it just means we'll have to be even more determined in the second leg.
"As Udinese have won the first leg, I'd say they have a 51% chance of qualifying."
The second leg will take place at the Stadio Artemio Franchi on February 11.