Chill out, Cowboys fans ... 'cause rookie backup QB Dak Prescott will have NO PROBLEM filling in for injured star Tony Romo, despite his inexperience ... his old H.S. coach says.
TMZ Sports spoke with Rodney Guin -- Prescott's football coach at Haughton High in Louisiana -- who says the rest of the world might be shocked by D.P.'s play ... but he ain't.
FYI -- Prescott's played at a RIDICULOUS level through the first 3 preseason games ... racking up 7 TDs (5 passing, 2 rushing) and 0 INTs on more than 500 total yards.
“Dak always prided himself in being ready to play," Guin told us. "He was the most prepared guy on the team ... he was a leader in practice, he earned the respect of all the older players and I can see him doing that on a professional level."
“I’m not really surprised by his great start. I always knew he would be successful, but it is impressive to see him play so well so early.”
In other words, maybe all hope isn't lost for Cowboys fans ...