Harbaugh didn't go into specifics on Speight's injury, but he did say John O'Korn will start in the Wolverines' rivalry game against Michigan State on Saturday in Ann Arbor and Brandon Peters will be his top backup.
Speight exited Michigan's 28-10 win over the Purdue Boilermakers on September 23.
"John's done a fantastic job ever since he got here. I thought it was egregious", Harbaugh said last week.
The 4-0 and No. 7 ranked Wolverines will look to remain unbeaten when the host (3-1) Michigan State this coming Saturday under the lights. The junior quarterback was sacked in the first quarter, then was on the receiving end of a second, border-line late hit from Eddy Wilson. Harbaugh called the hit "egregious", but Purdue head coach Jeff Brohm said he felt it was a clean play.
"It meant a lot for me, even if I don't play the rest of the year, to be able to go out and beat my former coach", O'Korn said. The Wolverines' red zone offense was a concern heading into that game with one touchdown on 10 attempts.
Though Speight was named the starter at the beginning of the season, the offense has had its well-documented share of struggles, particularly in the passing game. He was completing just 54.3 percent of his passes for 581 yards, 3 TDs and 2 INTs.