The 27-year-old will miss pre-season as well as the first half of the 2025-26 campaign after tearing his ACL in the 2-0 defeat to Crystal Palace back in February.
Martinez would have likely been a feature for Amorim through until the end of the season but will now have to work his way back into the first team.
Speaking to Clank Media, the World Cup winner was full of praise for his manager despite the lacklustre start to his Man United career.
He said: "The club is obviously going through a process of change. Since Six Alex Ferguson's departure, they haven't found a manager who hits the right note. Ten Hag did a remarkable job at Ajax, which is why he went to United, and thanks to him, I joined the club as well.
"It's like everything in life: you take a gamble, and sometimes it works out well, sometimes it doesn't. I think that, like any club, if something isn't working, it needs to be changed. It's necessary to analyse what's working and what isn't and make changes accordingly because that's how great clubs operate. Manchester is one of the best clubs in the world.
"Now Ruben Amorim has arrived with very good energy and willingness. I believe he will make the necessary changes. Without a doubt change is needed, because where we ended up in the table over the last two seasons is unacceptable."