Fausto Rossi's 17th-minute strike proved to be enough for Pucela to secure a shock 1-0 win over the Catalans on Saturday to leave the defending champions lying in third.
Speaking after the game, Zubizarreta could not hide his disappointment at the result but stressed that the Blaugrana still had every opportunity of seizing the La Liga crown this term.
"We have to take the defeat on the chin," the 52-year-old told reporters.
"We have to reflect now on the key moments of the season. It depends what happens - if Real Madrid win [against Levante on Sunday], we will be four points behind and it wouldn't be in our hands.
"Today we leave with a sour taste in our mouths."
The setback at the hands of Valladolid, which was Barca's second defeat in three games after they fell to a 3-1 reversal against Real Sociedad in late February, also allowed Atletico Madrid to return to the top of the table following a 2-0 win over Celta Vigo.
"It's a very long competition and it's not lost in one day or after one game," the former goalkeeper added. "We will learn from this defeat and will fight to win the coming games."
Barcelona are now third in La Liga with 63 points from 27 games, a point behind both Atleti and Real, though the Blancos currently have a game in hand over both of their title rivals.