Barca top the standings with 87 points, just one ahead of Real Madrid with four games to go, and must bounce back quickly after being dumped out of the Champions League at the semi-final stage by Jose Mourinho's Inter Milan side on Wednesday.
"The players are down and sad but we have to pick ourselves up. On Saturday we have La Liga and we have to go out and win," Guardiola said after their defence of the European Cup ended.
Barca's aura of invincibility is slipping after winning six trophies last year and they only have the league left to focus on now.
They have a harder run-in than their arch-rivals Real, starting with a trip to a side who have a knack of making life difficult for them.
Unlike Inter or Espanyol, who build a wall in front of the goal, Villarreal have been effective at taking on Barca at their own game, winning five of their last seven meetings at the Madrigal.
The hosts have recovered since Juan Carlos Garrido replaced Ernesto Valverde as coach in February and have climbed back into contention for a place in Europe next season.
They lie sixth with 52 points in the Europa League qualification places, four behind fourth-placed Real Mallorca and the Champions League slots.
REASONS TO BELIEVE
Real will be relieved knowing Barca will not be playing the Champions League final at the Bernabeu, and have been given more reasons to believe the leaders could trip up before their own match at home to Osasuna on Sunday.
Manuel Pellegrini's side have won all but one league match at the Bernabeu this season and are supremely confident Barca will stumble on the run in.
"We have a strong chance of winning the league," Brazil midfielder Kaka told a news conference.
"The dressing room believes we can do it. One point isn't much of a difference and we are confident we can overhaul Barca if we win all our remaining games."
Kaka scored on his return from injury as a substitute last weekend and is likely to start against Osasuna with both Rafael van der Vaart and Raul out with injury.
On Sunday, fifth-placed Sevilla receive Atletico Madrid in a dress rehearsal for next month's King's Cup final between the two sides.
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