Mengden was optioned to Triple-A Las Vegas on Monday, Julian McWilliams of The Athletic reports.
The move is something of a surprise, as Mengden seemed to have a good shot at the Athletics' fifth starter job. He could still win that job, as Oakland's unusual start to the season (an early two-game series against the Mariners in Japan on March 20-21 followed by six days off) means the team may simply want an extra hitter or reliever on the roster until a fifth starter is needed.