Vinicius Junior faces a spell on the sidelines after Real Madrid confirmed the Brazilian teenager has torn ligaments in his right leg.
The winger sustained the injury in the first half of Madrid's Champions League defeat at the hands of Ajax on Tuesday.
He was substituted in the 35th minute, with Ajax leading 2-0, and the Eredivisie side went on to win 4-1 and dump the three-time defending champions out in the round of 16.
Vinicius has been a rare highlight for Madrid in a terrible season that has seen the club dismiss Julen Lopetegui, with Santiago Solari reportedly close to following suit.
He has made 15 LaLiga appearances in 2018-19, scoring two goals, while he has also featured in four Champions League matches.
Madrid gave no timescale for his recovery in a statement released on Wednesday.
"After the tests made today to our player Vinicius Jr by the Medical Services of Real Madrid, he has been diagnosed with a torn ligament of the tibiofibular joint of the right leg. Evolution pending," the statement said.
Solari's future is under severe threat with Madrid 12 points off the top of the LaLiga table after Saturday's 1-0 home Clasico loss to leaders Barcelona.
Madrid were also knocked out of the Copa del Rey by Barca at the semi-final stage, losing 4-1 on aggregate, although Solari led the team to Club World Cup glory last year.