WASHINGTON (AP) The Washington Nationals say an MRI Stephen Strasburg underwent Monday morning has confirmed their diagnosis that the pitcher has an upper-back strain.

Strasburg threw a bullpen session Tuesday afternoon before Washington was set to play the New York Mets. Strasburg was placed on the disabled list Sunday retroactive to June 16.

Manager Dusty Baker said the MRI made him feel better and that he prayed and knew Strasburg was going to be all right.

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The 27-year-old right-hander was scratched from his last two starts with the back injury. Strasburg is 10-0 with a 2.90 ERA this season and signed a $175 million, seven-year contract extension in May.

With Strasburg on the DL, Washington turned to top pitching prospect Lucas Giolito to make his major league debut Tuesday against the Mets.