Walter was recalled from Triple-A Worcester prior to serving as the primary pitcher Thursday against the Twins, Chris Cotillo of The Springfield Republican reports.
The 26-year-old lefty is in line for his big-league debut and is scheduled to follow opener Justin Garza. Walter hasn't lived up to expectations this season at Triple-A (6.28 ERA, 1.65 WHIP), but he has been solid in his two most recent starts, in which he walked two and struck out 13 over 10.1 innings.