Mainoo sustained the injury in training on Saturday.
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"He got an injury in the last training," said Amorim. "After training, he went to the doctor. He got something in his calf.
"We are going to assess but he is out of this game.
"Of course (it is bad timing), not just for the team but for him to have this problem. These are reminders that we need to focus on everything we can control and leave the rest to the destiny.
"He is going to be fine in the few weeks, I think, I don't know. Let's focus on the players that we have and are ready to play."