Mignolet was on the Belgium bench on Tuesday as Fletcher's side lost 2-0, to leave their hopes of going to Brazil dangling by a thread after just four qualifiers.
But the Black Cats stopper expects his team-mate to bounce back after scoring five goals in his first six games for the club.
Mignolet said: "Steven was disappointed and shot off really quickly, while we were celebrating the win.
"I spoke to him when I got back and he said we deserved to win. It wasn't easy for him as a lone striker for Scotland against four or five defenders.
"Hopefully, he'll take his frustrations out on Newcastle, but I was worried at one stage when he went down injured.
"But it's not only up to Steven - the whole team have to get goals. We can't put too much pressure on him."