If Shawne Merriman were running the New Orleans Saints ... he tells TMZ Sports he'd sign Philip Rivers ASAP -- guaranteeing the QB is ready to play in the NFL today.
"One thing I know about Philip," said Merriman, who played with the quarterback in San Diego for years, "he's been tossin' the rock. There's no rust there."
Rivers has been linked to New Orleans after Jameis Winston tore his ACL in the Saints' game against the Buccaneers ... with the ex-Chargers QB saying this week he'd consider coming out of retirement if Sean Payton called.