The four-times Asian champions, who stunned Germany with a 2-1 win in their Group E opener, spurned a perfect chance to advance to the next phase on Sunday with a match remaining, conceding late on to a team that scored with their first shot on target of the Qatar World Cup.
Japan looked sluggish, static and out of ideas on how to beat a Costa Rica side that went for a rigid back five to prevent more goal haemorrhaging.
Moriyasu seemed reluctant to utilise his glut of attacking options on his bench.
"I don't think it was a total mistake from us," he told a news conference.
"Against Spain we are going for the win and that's what we need to focus on.
"The next match against Spain will be a very intense game. We need to improve the odds for Japan," Moriyasu added.
Japan have been a permanent fixture at the World Cup since debuting in 1998 and have reached the last 16 three times, more than any other Asian side.
The Samurai Blue can still qualify for the next round, but may need to pull off another feat of giant-slaying when they meet Spain on Thursday, a scenario made far less likely by Sunday's tepid showing in Al Rayyan.
Moriyasu said he would approach the Spain game differently and analyse how they performed against Germany in Sunday's later Group E match.
"We will of course come up with a different game plan, different tactics, of course for the next match we are going for a win," he said.
"Spain are a strong team but we were always thinking about three matches; we will try to advance, so we need good preparation."
"We have respect for them. Spain are a young team but I believe there's a good chance we can win; with confidence, we will go for the next match."
Check out Japan's game summary against Costa Rica with Flashscore.
