The most logical explanation is that they were intent on lulling the opposition into a false sense of security.
That is the only way to comprehend an abjectly-pedestrian first-half display -- from both parties, to be fair -- that would have left the 41,479 present wondering what all the fuss about a World Cup was about.
But Japan were the bigger culprits to blame for the sheer dreariness. They had just claimed a massive come-from-behind 2-1 win over Germany four days ago, coming into a game against opponents who had been thrashed 7-0 by Spain and knowing that victory would almost guarantee them a place in the Round of 16.
For all the possession they boasted, they hardly looked interested in finding an alternate route to goal when the repeated wing-play failed to reap the desired results.
And even if the gameplan had indeed been to bore Costa Rica into submission (it obviously wasn't), the big problem?
Japan ended up putting themselves to sleep, and ultimately were hit with the archetypal sucker punch when Keysher Fuller pounced on a defensive lapse with nine minutes remaining to score and hand the Costa Ricans a 1-0 triumph.
So after all the good work they achieved by beating Germany, plenty of that has been undone and the Japanese now have the unenviable task of needing a result against Spain on Thursday if they are to advance into the knockout round.
The players will certainly have to hold their hands up for a display that desperately lacked urgency and invention.
Still, Japan coach Hajime Moriyasu has to shoulder the main responsibility with some peculiar team selections.
Promoting Doan off the bench was understandable given he had bagged the equaliser against Germany as a substitute, yet the inclusion of Ueda and Soma -- both with less than ten caps to their names respectively -- was more difficult to rationalise, especially with the likes of Kaoru Mitoma, Takuma Asano and Takumi Minamino waiting in the wings.
The trio of Doan, Ueda and Soma -- along with playmaker-in-chief Daichi Kamada -- spent the entire first half resembling a quartet that had not played together much at all before. The truth is they indeed have not.
While the engine room of Wataru Endo and Hidemasa Morita kept winning possession in the middle of the park, any prospective attack broke down within seconds as the Samurai Blue struggled to string more than three passes together in the final third.
The changes eventually came after the break with Moriyasu opting to switch to a back-three, the same move that led to their revival against the Germans.
Strangely, again, instead of Takehiro Tomiyasu, who performed admirably four days earlier, it was Hiroki Ito who came on as the extra centre-back only for him to constantly give possession away with some untidy long passes that bordered on being ludicrous attempts.
It took until after the hour mark for Moriyasu to bring on Mitoma, arguably Japan's most-exciting player, and it did not take long for him to start causing Costa Rica problems down the left.
Unfortunately for Japan, he was fighting a lone cause. Too many around him had already been lulled into lethargy from their own doing.
Instead, it was Costa Rica who managed to find that extra bit of life in them at the end to get the win.
Japan will now have to awaken from their slumber -- and it should not be too difficult given the rude shock that awaits in the form of Spain -- lest they want their World Cup campaign to come to an end in the group stage.