Kelly was optioned to Triple-A Pawtucket on June 2 and injured his right groin a few days later.
"He threw a 40-pitch bullpen today," said Farrell. "He'll throw another one on Monday and we will get him back into games hopefully by mid-week some point and he'd come back as a reliever, not a starter."
The plan is for Kelly to pitch once in relief at Class A short-season Lowell and once for Double-A Portland and probably rejoin the Red Sox after the All-Star break.
The role is not a completely foreign one to Kelly. Though all 35 games he's pitched for the Red Sox have been as a starter, Kelly made 30 relief appearances for the Cardinals from 2012-13, going 2-3 with a 3.25 ERA over 52 2/3 innings.
"Power arm," said Farrell. "With the potential of the stuff playing up over shorter stints and him possibly being more productive, that's the whole goal here, and to also to continue to build depth in our bullpen."
Farrell discussed the move with Kelly by phone.
"Yeah, he was receptive, he understood the reasons for it, the logic behind it," said Farrell. "It's something he's done in the past. I think, more importantly, he's feeling good physically and that's the biggest thing. His bullpen today was a productive one, it was crisp. And the fact he has the potential of putting himself in a position to be a candidate to come up here sooner rather than later because of the role, that's another positive in this one."
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.