Barca at Real Madrid's mercy now, says Guardiola
The loss left Real with the chance to open up a 10-point gap at the top if Jose Mourinho's side beat Levante on Sunday.
"Win the title? Well, we will try to defend our Champions League crown and then attempt to negotiate each match in La Liga," said Guardiola on a freezing night in Pamplona.
"We will see what happens. But we are a significant distance behind which I guess will increase on Sunday. The league will be more difficult for us now.
"In the second half tonight we did well on this pitch but we should have done better in the first half. It was hard for us to get into this game but we have to congratulate Osasuna."
The reigning La Liga and Champions League winners have struggled on their travels this season with just four wins and succumbed to their third defeat on Saturday.
A double from Dejan Lekic put Osasuna in a commanding position before the break and while Alexis Sanchez reduced the arrears after 50 minutes, a clinical finish from Raul Garcia restored the two-goal cushion.
Substitute Cristian Tello hit Barcas second and there were chances at either end in the final stages but Osasuna held on.
While Barca struggle, Real continue to roll over their opposition, having won 18 of 21 games, scoring 71 goals in the process, eight more than the champions.
Guardiola continued to put his faith in youth against Osasuna and left Xavi Hernandez, Andres Iniesta and Cesc Fabregas on the bench with possibly their midweek Champions League match with Bayer Leverkusen in mind.
Sergio Busquets was also absent through injury, and the Catalan side lacked their normal fluidity on a bumpy pitch.
A double from Lekic put Osasuna in a commanding position before the break with only a shot from distance by Lionel Messi, tipped over by the keeper Andres Fernandez, offered in response.
Barca started the second half strongly and after five minutes substitute Juan Cuenca crossed for Alexis Sanchez to reduce the arrears.
But just as the visitors appeared to be turning the game around, Raul Garcia restored the two-goal cushion with a clinical finish off the post.
Barca had more of the ball and another substitute, Tello, fired in their second which beat the keeper at his near post.
In the final stages, Francisco Punal hit the post for the home side while Sanchez had a goal ruled out for offside.
Despite Barcelona's problems, Real Madrid coach Jose Mourinho feels that the title contest will not be decided imminently.
"February is possibly more difficult for us than Barcelona as we take on Levante, Racing (Santander) who are improving, Rayo (Vallecano), without doubt the surprise package this season, and then Espanyol, who are fighting for a place in Europe," he said.
"Levante are a good side and not just for the fact they have beaten Barcelona but also for where they are in the table. In October and November everyone thought they would run out of steam."
Earlier, a series of spurned chances saw Atletico Madrid draw 0-0 at struggling Racing Santander but the revival under Diego Simeone continued as they push for a place in Europe.
Atletico are now in sixth place while they are unbeaten in six games and yet to concede a goal under the new Argentine coach although this was their second consecutive goalless stalemate.
They are a point above Athletic Bilbao who lost 2-1 away to Real Betis after conceding an injury time strike from Nelson.
Ruben Castro put the home side ahead after 10 minutes but Athletic drew level with Javi Martinez heading home after being left unmarked at a corner.
Betis hit the woodwork either side of the break through Jefferson Montero and Castro and then really piled on the pressure after Martinez was dismissed after 64 minutes for a second yellow card.
They got their reward at the death through full-back Nelson who powered in a well struck shot from 25 yards.
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