The board of directors of the Pamplona-based club took the decision after the team's poor start, which has seen Osasuna lose their opening three matches to be at the bottom of the table, reports Xinhua.
However, it was not a surprise for the struggling club which sold leading goal-scorer Kike Sola during the summer and his replacement, Oriol Riera, spent last season with second division side Alcorcon.
Osasuna have lost at home 1-2 and 0-3 to Granada and Villarreal and 0-2 away to Athletic Club Bilbao. Club President Miguel Archando said that he had felt 'ashamed' during his side's defeat to Villarreal at the weekend.
Mendilibar leaves the club after just two and a half seasons: his first two years saw Osasuna punch considerably over their weight as one of the club's with the lowest budgets in La Liga, but hamstrung by debts of around 50 million euros, they struggled to 16th place last season and had looked likely to go down for much of the campaign.
Given the lack of spending and Sola's departure over the summer, this was always going to be a difficult season, but the lack of patience shown in Mendilibar, who had previously worked with Valladolid and Athletic Club among others, is surprising, especially as Archando insisted that despite speaking to potential candidates, the club still doesn't have a replacement lined up.
Mendilibar was a popular and respected figure among Osasuna fans and had received the support of several supporters during the training session in the morning prior to his sacking.
--IANS (Posted on 03-09-2013)