Ryan Giggs promised Wales would improve after their Spanish lesson, even though Gareth Bale is set to be absent again for the Uefa Nations League trip to the Republic of Ireland.
Wales were no match for a rampant Spain side as the visitors ran out 4-1 winners in a Principality Stadium friendly on Thursday night.
Real Madrid star Bale was missing with what Wales manager Giggs described as "muscle fatigue", and the chances of the 29-year-old forward playing in Dublin appear slim.
"Gareth is struggling for Tuesday," Giggs said after Wales' heaviest defeat for six years. "We will make a decision in the next couple of days.
"We will make it sooner, rather than later."
Asked if it looked at the moment that Wales would be without their headline performer, Giggs said: "Yes, it does."
Wales also have injury concerns over Chelsea midfielder Ethan Ampadu (knee) and Brentford defender Chris Mepham (glute) for Dublin.
There was plenty of pain on the pitch, too, as Wales ended a seven-and-a-half-year wait to play at the 74,500-capacity home of Welsh rugby.
A crowd of 50,232 saw Spain storm into a three-goal lead through Paco Alcacer and Sergio Ramos inside 29 minutes before substitute Marc Bartra added a fourth in the second-half.
Sam Vokes headed a consolation a minute from time to provide Wales with some cheer before their journey across the Irish Sea.
"It is disappointing," Giggs added. "Against average teams, you have to do the basics right.
"Against a very good team, if you don't do the basics, you get punished, and that's what happened.
"We brush ourselves down, take it on the chin. The last time we were in Cardiff we won 4-1 and were magnificent.
"The good thing about football is you have always got the next game, and that can't come quickly enough.
"We will be better on Tuesday."
Spain had a terrible World Cup when manager Julien Lopetegui was dismissed on the eve of the tournament after it was revealed that he was joining Real Madrid at the end of it.
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The former European and world champions bowed out at the last-16 stage, but Luis Enrique has revitalised them in his short time in charge.
Spain have scored 12 times in his three games and approach their Nations League return with England in Seville on Monday full of confidence.
"It was an intense game and I loved watching my team," Enrique said.
"The team were very involved from the first minute and had an almost perfect attitude."
"Yes, there are [selection] clues [for the England game] in terms of those who have not played tonight, but you won't be able to guess more than four or five [of the team].
"I love the competition. It means we can bring in four or five players and they still have a very high level."