Johnson (elbow) will make his second rehab start Tuesday at Double-A Portland, Alex Speier of The Boston Globe reports.
Johnson began his rehab assignment six days earlier with Triple-A Pawtucket, tossing 1.2 innings and giving up two runs on three hits and a walk. He'll likely work no more than three frames Tuesday while preparing to rejoin the Red Sox in a multi-inning relief role once activated from the 10-day injured list.