Vinicius Junior will be sidelined until the tail end of the season after Real Madrid confirmed the Brazilian teenager has torn calf ligaments in his right leg.
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 La Liga 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 but reports have suggested he will miss the next two months.
"After the tests made today to our player Vinicius by the medical team of Real Madrid, he has been diagnosed with a torn ligament in his right calf. Further evaluation pending," the statement said.
The injury to Vinicius will also come as a blow to his international aspirations, with the 18-year-old having been called up to the Brazil squad for the first time.
Madrid head coach Solari, meanwhile, is under severe threat of losing his job, with his side 12 points off the top of the Liga table after Saturday's 1-0 home Clasico loss to leaders Barcelona.
The Blancos 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.
It remains to be seen if Solari will still be in charge by that point, with the likes of Jose Mourinho and Mauricio Pochettino having been linked with the top job at the Bernabeu.