Cobb took the mound before Friday night's game against Toronto.
''Huge day,'' he said. ''Felt great. It's the first major step.''
Cobb has no date for when he will rejoin the rotation, a process that will eventually require at least several minor league rehab starts.
Cobb was going to start opening day but instead began the season on the 15-day disabled list. He went 10-9 with a 2.87 ERA in 27 starts last season.
First baseman James Loney, coming back from a right oblique strain, was also reinstated from the 15-day DL. Outfielder Mikie Mahtook and first baseman Allan Dykstra were optioned to Triple-A Durham after Thursday night's 2-1 win over Boston.