Munoz arrived at the tournament carrying a calf issue and had been following an individual recovery programme in a bid to regain full fitness. The 22-year-old appeared to be making encouraging progress and was named on the bench for Spain's goalless draw against Cape Verde in their opening Group H fixture.

However, the Royal Spanish Football Federation (RFEF) announced on Friday that the former Osasuna winger had sustained an additional muscle injury during his rehabilitation, casting serious doubt over his involvement in the remainder of the group phase.

"During the planned and individualised recovery process, an additional muscle injury has occurred that will delay his return to competition," the RFEF said in a statement.

"His availability for the upcoming matches will depend on the evolution of his symptoms."

The setback rules Munoz out of Sunday's clash against Saudi Arabia in Atlanta and leaves him highly doubtful for Spain's final group match against Uruguay later this month. Spanish media reports suggest it remains unclear whether he will recover in time for the knockout rounds should Luis de la Fuente's side progress.

The news comes less than 24 hours after Liverpool confirmed the winger's arrival in a deal worth around 34 million pounds, with the Premier League champions beating off competition from Newcastle and Manchester United to secure his signature. Munoz has signed a six-year contract at Anfield, becoming the first major addition under head coach Andoni Iraola.

Liverpool medical staff reportedly travelled to the United States before the transfer was finalised to assess the player's fitness, although the latest development means supporters may have to wait longer to see their new signing in action.

Spain will now look to bounce back from their surprise stalemate with Cape Verde without Munoz, with Lamine Yamal and Nico Williams expected to play more prominent roles after both were used from the bench in the tournament opener.

Despite the excitement surrounding his move to Merseyside, Munoz's immediate focus remains on recovery. Following his departure from Osasuna, the winger issued an emotional farewell message thanking the club, his teammates and supporters for what he described as the most enjoyable period of his career.

"It's never easy to say farewell to a place where you've been very happy and where you lived the best footballing year of your career," Munoz said.

"I always tried to give everything on the pitch and give back all the love I received. I'm leaving, but I'm taking away much more than football. I take with me friendships, lessons and people who will be part of my life forever."

For now, both Liverpool and Spain can only wait and hope that the talented winger's latest setback does not bring a premature end to a landmark month that was supposed to be defined by a dream transfer and a first World Cup appearance.

Ronwen Williams nominated for top goalkeeper award