The 22-year-old has been enjoying a prolific spell with the Silkmen, plundering 16 goals in 18 games, but has had to return to Glanford Park for two operations on a calf injury.
"He had his first operation on Friday and it's a problem in his calf area," caretaker boss Russ Wilcox told the club's official website. "I don't want to go into too much detail because I'm not an expert in those lines, but it is a bad injury he has received.
"He had another operation on Saturday and I'll speak to him again on Sunday.
"It started as a dead leg. That's something your body normally filters away.
"It hasn't done that and that's when it can become a problem; it can become a blood clot. What they've had to do is cut into the calf to release the pressure."
"It's been a serious operation and an injury that they've had to look after very quickly."
Jennings, who had been at Moss Rose since the start of the season, was due to return to the Iron in January, and after being so prolific in the Skrill Premier, Wilcox was looking forward to having at his disposal.
"It's just a bad injury that's really disappointing - for everybody really - ourselves, Macclesfield and more importantly Connor himself," he added.
"We just wish him a speedy recovery, we will assess him, see how he is and he will come back to us for his rehab because he is our player."