Everton's Bryan Oviedo suffered a double leg break during his side's FA Cup fourth-round tie at Stevenage.
The Costa Rica international was hurt during the first half of his side's 4-0 win and needed lengthy treatment on the pitch before he was taken straight to hospital.
Several of Oviedo's team-mates were in obvious distress after witnessing the incident, and Roberto Martinez confirmed the extent of the injury in his post-match press conference.
"The injury to Bryan was very unfortunate," he said. "It wasn't malicious, Bryan is trying to win the ball back and unfortunately has come away with a double fracture, a break in both the tibia and fibula.
"We are going to assess now what is the next step for Bryan. It is one of those moments that really gives you a bad taste.
"After the injury, the players had to refocus because Bryan is a nice human being and an incredible professional. When something like this happens it gives you a really bad feeling.
"With his level, he has shown the quality that he has and the player that he can be. You feel terribly sad for him because it is a great moment in his career, with the World Cup around the corner.
"All we want is Bryan recovered as quick as we can so that he comes back as important as he has been for the squad.
Everton later issued an update on their Twitter page. It read: "Bryan has broken two bones in his left leg. He will remain in hospital over the weekend and have an operation on Monday."
Ross Barkley and Darron Gibson also tweeted supportive messages and Oviedo, whose World Cup participation must now be in question, was later well enough to post his own thoughts.
The 23-year-old, who began the Stevenage game in midfield but has also played left-back, wrote: "Thank you everyone at this difficult moment. You all are in my heart."