The winger is certain the former Chile coach will remain with the Basques, and hopes that Spain's Olympic campaign will be a successful one
The Argentine recently submitted his resignation to the Basque side after Athletic refused to stand by him following a dispute with a construction company employee over work that was not yet completed at the club's training ground.
The coach, who had recently signed a one-year contract extension in June, later reversed his decision to leave, and Muniain has quashed further suggestions the 56-year-old is set to depart.
"It might have been a mess, but everything is back to normal," Muniain told Europa Press. "Today, Bielsa is the coach here, and it will stay like that."
Speaking ahead of the London Olympics, Muniain also affirmed he had used his break to recuperate after a demanding season, which saw his side bow out of the Europa League and Copa del Rey finals.
"It was a tough season, but I personally used the break to the fullest to recharge my batteries," the 19-year-old said to AS.
"There were many games, some of a very high level, with little break in between, but I'll be ready for Spain's opening match against Japan.
"At first glance, people may think it's an easy group, but all the sides [Japan, Morocco and Honduras] present deserve to be there. All three opponents are at a high level, and everyone here has to do their part in these tough games."
Spain start their Olympic campaign against Japan on July 26 at Hampden Park.