Johnson will replace left-hander Brian Matusz on the 25-man roster. Matusz was optioned after today's 6-2 loss to the Indians at Camden Yards.

Johnson, 24, is 3-5 with a 2.73 ERA in 14 games, including nine starts. He's allowed 43 hits, walked 24 and struck out 63 in 62 2/3 innings, and opponents are batting .194 against him.