Middlebrooks is eligible to come off the 15-day disabled list on Saturday, though an extended stay in the Minors could be beneficial.
"We didn't set a timeline on when his return would be," Farrell said on WEEI. "We want to make sure he comes back to us on a little bit of a roll.
"The one thing we've outlined clearly to Will is that this rehab assignment is not only to make sure his rehab issue is behind him and to get his timing back, but also to make sure he gets into a good place as far as offensive production."
Farrell said he thought Middlebrooks was expanding the strike zone early in the season, which is part of the third baseman's aggressive approach, though it can be detrimental if he isn't connecting more frequently.
While Middlebrooks struck out almost as frequently his rookie year, fanning 70 times in 267 at-bats, he also hit 15 homers with a .288 average. This season, he's struck out 54 times in 174 at-bats while posting a .201 average.
"I thought he stayed within a defined strike zone more regularly after the rib injury [in May]," Farrell said.
The Red Sox have also been challenged against left-handed pitching this season. Entering Wednesday, they were tied for 20th in the Majors with a .241 average against lefties while ranking first with a .288 average against righties. A healthy, productive Middlebrooks could add a dangerous threat against southpaws.
Farrell also announced that Jacoby Ellsbury could need a stint on the 15-day disabled list due to a tight left groin. Tuesday, Farrell said it appeared unlikely that Ellsbury would go that route, but Wednesday presented a change of thought from the manager.
Shane Victorino will begin a rehab assignment for his ailing left hamstring and sore back on Thursday, Farrell said.
Jason Mastrodonato is a reporter for MLB.com. Follow him on Twitter @jmastrodonato. This story was not subject to the approval of Major League Baseball or its clubs.