The 5-foot-9, 159-pounder scored two second-half touchdowns, one on a reception and another on a diving run, and also contributed at defensive end to help halt the visiting Cavaliers' first-half dominance.
After Sheffner's six-yard scoring run shaved what once was a 19-point Santiago lead to just five with 7:02 left, the Mustangs forced consecutive three-and-outs to get two chances at a game-winning touchdown.
But a 54-yard punt pinned the Mustangs at their own 11-yard line with 4:52 left and they produced nothing.
Mesa regained possession on its own 33 with 3:06 remaining and moved to the Santiago 45. But the Cavaliers' fourth sack of the game forced a fumble that defensive end Chris Montes recovered to squelch the Mustangs' comeback dreams.
"It was a gut check," Osso said of the turnaround. "We challenged our players at halftime and told them it was either time to put up or shut up. They did a great job in the second half of putting up."
Santiago (1-0) put up all its points in the first two quarters, scoring on its first four possessions and not punting until the final five minutes of the game. The Cavs had a pair of nine-play touchdown drives and another of 11 plays to nearly double the Mustangs' first-half time of possession.
An 86-yard kickoff return for a touchdown, after Mesa quarterback Nathan Alvis hit Trace Curet for a 21-yard scoring pass with 4:48 left in the first quarter and the first of three Peter Marquez PATs put the hosts up, 7-6, turned out to be a difference maker.
"If we don't give up that kickoff return, we win," Osso said.