Martinez was stretchered off during Manchester United’s 2-0 loss to Crystal Palace.
The Argentina international suffered an awkward twist while challenging Ismaila Sarr in the second half at Old Trafford.
Martinez immediately signaled for a substitution, visibly in pain as he was carried off to receive treatment for the injury.
"I think it is a serious situation," Amorim stated post-game to reporters.
"A bad situation for us and especially for Licha.
"I think we are going to see in the next days but I think it is a serious situation. It's really hard on us, he's not just a great player but a strong character in the dressing room. Especially in this moment, it's hard for us.
"Now it is time for all of us to help Licha. He felt it and, when you are a player, you know when it is serious.
"Let's hope for better days for him. We are here, like I say, to help him in this difficult moment."