The Denver Broncos quarterback has used the call over 70 times in his side's last two games, but it has been widely recognised that no two plays appear the same.
Sherman told reporters: "I feel like there's no Omaha code. I feel like he might be just throwing it out there. He says it so much, it has to be talking to one player or something crazy like that. I don't know if we'll ever be able to break the Omaha code."
Teammate Thurmond added: "It's to get defenders second-guessing themselves from what's actually going on instead of just playing football. A lot of that talking is just for show. There's no need to panic because he changed something.
"Well, you don't know what the play was in the first place. What do you think they're going to change to? It's mind-boggling how that situation influences a lot of players."
Manning broke records for both passing yards and touchdowns thrown this season. He has one career Super Bowl victory to his name, winning the title in 2007 with the Indianapolis Colts.