It is, some may say, just the Olympics.
It's not a fully senior tournament and the presence of the overage players means it's not a full youth tournament either, and that means the seriousness with which countries take it is never entirely clear (and is often amended according to results). But still, given Spain's overwhelming success both at senior and youth levels, there is something disconcerting about seeing them not merely beaten but outplayed.
The way Japan's Yuki Otsu broke down at the final whistle, tears pouring down his face, made clear just how seriously he, at least, takes Olympic football: This was the first time Japan had ever beaten Spain at any level and that clearly means something.
Spain, of course, won their opening game neither in the Euros, when they drew 1-1 with Italy, nor at the World Cup, when they lost to Switzerland. But this was different. This wasn't a game in which they were held at arm's length, or a freakish game in which Spain outchanced their opponents by a factor of eight or nine, as they had against Switzerland. This was a game in which they were clearly second best. Spain won the European Under-21 championship last year. They've won the European Under-19 championship in each of the last two years. They lost on penalties to the eventual winners Brazil in the Under-20 World Cup at the quarter-final stage last year but they were arguably the best side in that tournament. An endless dynasty of success seemed possible.
One set-back, particularly one with ten men, doesn't invalidate all of that, of course, but the defeat to Japan did hint that the younger crop might not live up to the achievements of the present generation.
Spain's performance at Hampden Park, in fact, served to highlight just how good their senior side is. The shape may have been a 4-2-3-1 with a fairly orthodox centre-forward in Adrian Lopez, as opposed to the strikerless 4-3-3 Vicente Del Bosque seems to prefer, but the style was very familiar.
The ball was worked around midfield, neat triangles denying their opponents the ball. There were a couple of longer passes hit on diagonals towards Lopez, but essentially this was the Spain of stereotype, intent on wearing their opponents down with ball retention. Juan Mata drew a diving save from Shuichi Gonda with a drive from the edge of the box but that was the only shot Spain mustered from an opening 30 minutes in which they were almost entirely dominant in terms of possession.
With the senior side that wouldn't have been a problem; playing the way they do, chances come. But this is not the senior side. It may pass the ball neatly but it lacks the sense of command of the full team. Japan had unsettled them with occasional presses - sustained in the first 10 minutes , more infrequent after that - and then with rapid counter-attacks led by the sprightly Kensuke Nagai. And then Japan took advantage of some abysmal defending to score.
Takahiro Ogihara's delivery from a right-wing corner was good, but David De Gea confused his defence by starting to come then stopping, and Martin Montoya was sloppy in allowing Yuki Otsu to muscle in front of him to prod a half-volley past the retreating De Gea.
It was subsequently startling to see how rapidly panic set in. Montoya played Alvaro Dominguez into trouble with a poor pass and his attempt to lay it back to De Gea let in Hiroshi Kiyotake, who slid the ball across the face of goal having sidestepped De Gea. And then panic became self-destruction. Again Japan's pressing forced Spain into an error, a poor backpass putting Inigo Martinez under pressure. He ended up pulling down Nagai and was sent off.
With the extra man, Japan began the second half superbly. It would have been easy for them to rush things, to try to force the game but they didn't, sitting off almost as Spain do. The chances for them to increase their lead came: Higashi drew a fine diving save from De Gea - his shot-stopping, as ever, better than his decision-making; Nagai stabbed an effort wide after being laid in by Higashi; and then Kiyotake shot wide after wrestling through.
The introduction of Oriel Romeu for Isco calmed Spain and gave them additional coherence. They began to use Mata and Rodrigo on the flanks again. Japan, though, remained admirably organised, their energy levels barely seeming to dip.
Spain created nothing clearcut and the chances kept coming for Japan. Nagai was denied against by De Gea, out quickly to block, and then, after Nagai had held the ball up superbly, Hotaru Yamaguchi fluffed his shot. It mattered only to the extent that there was always the possibility of Spain nicking something; it would have been a steal, though: Japan were fully deserving of their win.
None of the three favourites had easy rides. Uruguay had to come from behind to beat UAE - another tactical triumph for their veteran coach Oscar Washington Tabarez - while Brazil had to see off a second-half fightback from Egypt that turned a 3-0 half-time lead into a 3-2 final score. Even in the first half, Egypt looked good in possession but they were open at the back and Brazil took full advantage. The new Chelsea signing Oscar set up the first two with intelligent passes - his ability to see the right option may be his greatest asset - before Neymar added a third after a one-two with Hulk.
At the front at least, they were exceptional. At the back, rather less so. And defending has been the coach Mano Menezes's problem since at least last year's Copa America: They struggle to get the balance right between front and back.
Brazil may still be a work in progress, but they're in a much better position at this stage than Spain.