Flexen is expected to follow opener Keynan Middleton against the Rockies on Wednesday, Ryan Divish of The Seattle Times reports.
Flexen lined up to start Wednesday's contest, but the Mariners will instead go with an opener. Flexen has pitched well over his past seven starts, lasting at least five frames and allowing no more than three earned runs with a 1.77 ERA, 0.94 WHIP and 32:9 K:BB across 45.2 innings.