Janssen has already pitched twice for Double-A New Hampshire, tossing scoreless innings on July 9 and July 11. He will throw another inning on Thursday night before resting on Friday and pitching on back-to-back days on Saturday and Sunday.
The two outings this weekend will serve as a litmus test to determine whether the 29-year-old is ready to return to the Majors.
"He has thrown the ball well in his first two outings," manager John Farrell said. "Considering the nature of the injury ... we wanted to be sure that we built in one back-to-back outing to answer that question before coming back."
If all goes well this weekend, Janssen would be in line to return on Tuesday for the start of a three-game series against the Mariners at Rogers Centre. But on Thursday the team was noncommittal as to when he would return.
"I would expect that to be the case, but I have to leave it open, because you never know what's going to happen," general manager Alex Anthopoulos said of a Tuesday return. "But Casey was one of the better relievers at the time he went down, so I certainly can't see how he wouldn't help this team by being back up here."
Janssen was 2-0 with a 2.93 ERA and 25 strikeouts in 27 2/3 innings before his injury.