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It was the 11th time in the first 70 games of the season Ortiz was not in the starting lineup.
"We've given him a day down, because physically I think he needs it," said Farrell.
Ortiz is hitting .339 with 18 homers and 59 RBIs this season.
"Every game that David starts, I'm always touching base with him in the middle innings to see how he's feeling. The heel and the feet, those are real situations that he's managing," said Farrell. "He's putting forth every effort with the training staff to get him ready, to keep him on the field. We also monitor the number of times he's been on base, particularly the doubles.
"The number of doubles he's hit this year, he's probably 40 percent higher than last year. When he hits the ball out of the ballpark, he's going to jog. Home to second or first to third, that's where it really starts to pile up on David."
One way Farrell has been able to rest Ortiz is by not playing him in any National League road games, when the Red Sox don't have the designated hitter.
Boston has six road Interleague games left. Three in Los Angeles from Aug. 5-7, and three in San Diego from Sept. 6-8. At this point, the only way Ortiz is likely to play another game at first base in his career is if the Red Sox make it to the World Series.
"Yeah, I don't see us having him at first base in those particular Interleague road games, unless something changes from what we've gone through already," said Farrell.
Ian Browne has covered the Red Sox for MLB.com since 2002. Follow him on Twitter @IanMBrowne and Facebook. This story was not subject to the approval of Major League Baseball or its clubs.