Mainoo missed last weekend's game against Aston Villa with a calf issue, while Fernandes was forced off at half-time of the 2-1 loss at Villa Park after picking up a soft-tissue problem in the first half.
Amorim had suggested previously that both were likely to be out for the Newcastle match and, despite confirming that belief, he doesn't believe either midfielder will be out for long.
"No, not for this game," the boss said in the first part of his pre-match press conference on Friday afternoon, when asked if Fernandes and Mainoo would be available.