Sampson, who will report to Triple-A Tacoma, went 8-8 with a 3.98 ERA in 21 starts for Triple-A Indianapolis, leading the International League in starts and ranking third in innings pitched (124 1/3) and fourth in strikeouts (95).
Happ was 4-6 with a 4.64 ERA in 21 games (20 starts) with the Mariners after he was acquired last winter from Toronto for outfielder Michael Saunders. Happ will be a free agent at the end of the season.
Happ's spot in the rotation will need to be filled before his next scheduled outing Tuesday in Colorado, which general manager Jack Zduriencik said would come from within the organization.