Tanaka rebounded from a slow spring training start by allowing one run over four innings in Tuesday's game against Philadelphia. He had a procedure in October to remove a bone spur from his throwing elbow.
''He felt good today,'' Girardi said. ''He's confident he's ready to go.''
Tanaka will be followed in the rotation by Michael Pineda, Nathan Eovaldi and Luis Severino.
''Very happy to be given the position,'' Tanaka said through an interpreter.
Girardi said there is no decision yet on whether CC Sabathia or Ivan Nova will be the fifth starter.
''There's a lot to think about,'' Girardi said. ''When you have tough decisions, it doesn't always mean it's permanent. You have to prove yourself.''
Girardi said the choice will be made in the next couple days. The fifth starter will make his season debut a week from Saturday at Detroit.
''Just have to wait and see,'' Sabathia said. ''It's up to them. I'm ready to go. Just a waiting game.''
Sabathia said he had no idea why there was a delay in the decision. ''It's part of the game,'' he said.
Girardi said the loser of fifth starter competition will start the season as a reliever.
He also said Austin Romine will be New York's backup catcher. Luis Cessa and Johnny Barbato made the team as relievers, and right-hander Anthony Swarzak and outfielder Cesar Puello were reassigned to minor league camp.