Boston manager Terry Francona said before Saturday's 5-3 victory over the Mariners (17-30) that Ortiz would also sit out Sunday's finale after being relegated to a backup role for the first two games of this series.
Francona stressed that he's not benching Ortiz, who is hitting .208 without a home run in 144 at-bats dating back to last season, he just wants the five-time All-Star to clear his head and relax. The plan is for Ortiz to return to the lineup Tuesday when Boston (22-15) opens a six-game homestand against AL East-leading Toronto.
Without Ortiz on Saturday, the Red Sox hit three homers - their most since April 25 against the New York Yankees - to snap a three-game skid.
Jason Bay slugged his 11th homer of the season in the first, Jason Varitek hit a two-run shot an inning later and Jeff Bailey also connected to account for all of Boston's runs.
"We'll take them," Francona said.
Bay has been carrying Boston's offense with Ortiz struggling to find his stroke.
The power-hitting left fielder has six home runs and 20 RBIs in his last 12 games, and the Red Sox are 9-1 in games Bay homers.
With Bay leading the offense, Justin Masterson (2-2, 4.89 ERA) looks to help the Red Sox split their six-game road trip as he takes the mound in Sunday's finale.
After allowing six runs in each of his previous two starts - both losses - Masterson gave up two runs and five hits in six innings of a 4-3 loss to the Los Angeles Angels on Tuesday, but wasn't a factor in the decision.
He is 2-2 with a 4.97 ERA in five starts in Boston's rotation, and this will likely be his final start with Daisuke Matsuzaka (right shoulder strain) expected to come off the disabled list soon.
In Masterson's only career start against the Mariners on June 8, the right-hander allowed one run and three hits in six innings of a 2-1 victory.
Seattle lost for the 10th time in 12 games Saturday and is trying to avoid falling four games under .500 for the first time all season.
Russell Branyan had two of the Mariners' six hits Saturday and is 7 for 15 with a homer and three doubles in his last four games. The rest of Seattle's lineup is just 23 for 124 (.185) in those contests.
Jason Vargas (1-0, 1.04) looks to build off an encouraging first start of the season Sunday when he takes the ball for the Mariners.
In the rotation for an injured Carlos Silva (right shoulder inflammation), Vargas allowed one run and five hits in five innings of Tuesday's 7-1 loss to Texas, but didn't factor in the decision. It was Vargas' first start since July 3, 2007, after missing the 2008 season because of hip surgery.
"I felt pretty good from the beginning," the right-hander told the Mariners' official Web site. "I definitely got more in a rhythm after the second inning, but I really didn't feel off-balance or anything out there. I felt all right."
This will be his first career appearance against Boston.