The right-handed-hitting Jackson will compete with McDonald for at-bats against lefties.
McDonald has struggled this season, though his bat has picked up a little steam over the last two weeks, when he's gone for 6-for-20 with his fifth home run of the season, all of which came off left-handers.
Jackson, meanwhile, hasn't hit a homer off southpaws this year, though he his batting .254 with a .333 on-base percentage and 10 doubles in 122 at-bats.
"It's going to be a little bit interesting to figure that out," Francona said about how the two will split time. "That's my answer. I don't have a perfect answer right now. We certainly want to see Conor play, and it will probably cost Mac some at-bats, but we'll try to figure that out."
Jackson was eventually added to Friday's starting lineup in left field, as Carl Crawford was a late scratch due to illness.
Francona also said he'll try to rest Kevin Youkilis down the stretch, getting him some at-bats at designated hitter, which could lead to Jackson getting some time at third base.
Jason Mastrodonato is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.