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Seattle is sending five All-Stars to the Midsummer Classic for the first time since 2003.
RHP Luis Castillo (5-5, 3.31 ERA) opposes Tigers ace Tarik Skubal (10-2, 2.06) in the opener of a three-game series Friday at Detroit.
All-Star RHP Bryan Woo (8-4, 2.77 ERA) starts the series finale Thursday night for Seattle against RHP Marcus Stroman (1-1, 7.45).
Mariners rookie Logan Evans (3-2, 2.96 ERA) starts Wednesday night.
Frei was alert as he left the field, Sounders coach Brian Schmetzer said.
Right-hander Logan Gilbert (2-2, 3.40) is scheduled to start Tuesday at the New York Yankees.
Seattle RHP Logan Gilbert (2-2, 3.55) and Kansas City Noah Cameron (2-4, 2.79) start Wednesday in the third game of the four-game set.
Royals RHP Michael Lorenzen (4-8, 4.91 ERA) starts Tuesday against RHP Emerson Hancock (3-3, 5.43) in the second game of the four-game series.
The Mariners return from their second 10-day road trip in a month to face Kansas City on Monday.
Seattle hosts Columbus on Sunday.