The 19-year-old suffered cartilage damage during his side's Youth Cup defeat by Kilmarnock on Sunday.
Erwin, who missed four months of last season with a similar injury in his other leg, had just returned to Fir Park after scoring eight goals in 11 games on loan at Arbroath.
Motherwell manager Stuart McCall told the club's official website: "It's a real shame as we were looking forward to having him back in the fold after a real prolific run at Arbroath.
"That loan and playing first-team football was exactly what he needed, so it's a big blow for both the lad and us that he will miss such a large part of the season.
"The positive is that he has battled back from this sort of setback before and has shown a real strength of character."