Schreiber (3-0) earned the win Friday versus the Rangers, allowing two hits and two walks with one strikeout in a scoreless inning.
Schreiber (2-0) pitched a perfect inning and was credited with the win Wednesday over the Blue Jays.
The Red Sox traded Schreiber to the Royals on Saturday in exchange for David Sandlin, Jeff Passan of ESPN.com reports.
Schreiber arrived at Fenway South in Fort Myers (Fla.) early and has been working out since Jan. 11, Peter Abraham of the Boston Globe reports.
Schreiber signed a one-year, $1.175 million contract with the Red Sox on Thursday to avoid arbitration, Rob Bradford of WEEI.com reports.
Schreiber allowed one run on two hits over one inning and blew the save in Saturday's 4-3 extra-innings loss to Toronto.
Schreiber will serve as the Red Sox's opening pitcher for Saturday's game against the Blue Jays, Gabrielle Starr of the Boston Herald reports.
Schreiber (shoulder) was activated from the 60-day injured list Tuesday, Mac Cerullo of the Boston Herald reports.
Schreiber (shoulder) is expected to be activated July 23 versus the Mets or July 25 against Atlanta, MLB.com reports.
Schreiber (shoulder) will begin a rehab assignment this weekend, Mac Cerullo of the Boston Herald reports.