O'Neill, 61, could be forgiven for cursing his luck - Fletcher suffered ankle ligament damage while on international duty with Scotland last week - but is refusing to do so.
He said: "Of course it's tough, but that's the nature of the game. Football management now is probably more difficult than ever before because you are judged on any given 10-minute spell, never mind anything else.
"Remaining upbeat is actually in my nature. Yes, of course I can get down when we have lost matches, but fighting back is part of it.
"I was talking to a few Sunderland fans yesterday in a little forum and it was that downbeat nature - they didn't expect us to get through the international matches without some misfortune, so that might tell you.
"But it's an opportunity for other people and it's up to us to be raring to go."