WEST LAFAYETTE, Ind. -- John O'Korn has been patient yet emotional. The fifth-year transfer quarterback had no reason to complain about playing time with No. 8 Michigan off to a 3-0 start and Wilton Speight leading the offense.

But when Speight was injured during the first quarter Saturday after a hit to the head, it was O'Korn's turn to show what he can do as a quarterback for one of the nation's most tradition-rich programs.

O'Korn responded and led Michigan to a 28-10, come-from-behind victory over Purdue on Saturday in sold-out Ross-Ade Stadium.

"It was an emotional game for me, just for things to come full circle after everything I've been through," O'Korn said. "Moving forward, the quarterback situation is not up to me, even if I don't play the rest of the year. I will be ready if my number is called.

"Everybody knows that Wilton and I are good friends, and I hate that he had to get hurt for me to go in there.

"I am comfortable with the whole offense. The coaches have done a great job of keeping me in tune with the game plan every week and keeping me ready to play. It's cliche, but you always are one play away. It is a next-man-up mentality."

O'Korn, through Michigan's first three games, had completed his one pass attempt for 37 yards.

After Speight's head injury with 5:06 left in the first quarter, O'Korn led four touchdown drives, completing 18 of 26 passes for 270 yards and one touchdown. Speight was taken to a local hospital for X-rays but did return to Ross-Ade Stadium.

"I thought John really played great," Michigan coach Jim Harbaugh said. "He was seeing things really good right from the time he came into the ballgame.

"He ran the offense well and made big plays. I'm really proud of him and proud of the whole game. I learned today that we can win in the late rounds. That is a great thing to learn about your ballclub. You never know quite what the character is. Things get tough, and a lot of people start groping for alternatives. Our guys dug deep and finished them off."

Chris Evans' 49-yard touchdown run with 6:46 to play -- his second TD of the afternoon -- completed a 21-0 Michigan second-half blitz and helped the Wolverines (4-0, 1-0) remain unbeaten. Evans rushed 14 times for 97 yards.

"In the second half, Michigan beat us down the way Michigan beats people down," said Purdue coach Jeff Brohm, whose team gained 10 second-half yards and finished with 189. "In the second half, they were the better team. The second half was unproductive offensively. The better football team won."

Michigan had stretched its lead to 21-10 with 10:36 remaining on Ty Isaac's 1-yard run, capping a nine-play, 65-yard drive that included O'Korn completions of 23 and 14 yards that moved the ball to the Boilermakers' 3-yard line.

The Wolverines seized a 14-10 lead with 2:42 to play in third quarter on Evans' 10-yard run, completing a methodical 11-play, 86-yard drive that included O'Korn's 30-yard completion to Sean McKeon to the Purdue 37 and a targeting penalty.

Michigan took a 7-0 lead with 11:34 remaining in the second quarter on O'Korn's 12-yard pass to tight end Zach Gentry, capping a 13-play, 84-yard drive.

On the ensuing drive -- Elijah Sindelar's first after replacing starting quarterback David Blough -- Purdue (2-2, 0-1) drove 75 yards in five plays, pulling even at 7 on Sindelar's 10-yard touchdown pass to tight end Brycen Hopkins with 9:45 left in the half.

With 8:15 remaining in the second quarter, Boilermaker safety Navon Mosley intercepted O'Korn at the Wolverines' 27. J.D. Dellinger's 29-yard field goal with 6:04 remaining pushed Purdue into a 10-7 halftime lead.

The Boilermakers outgained the Wolverines 179-131 during the first 30 minutes, including 99 passing yards from Sindelar on 6 of 12 attempts. Blough was 3 of 7 for 11 yards during the four first-half drives he directed.

NOTES: Purdue rushing leader Tario Fuller (43 rushes, 261 yards in three games) did not dress because of an ankle sprain. ... Game time temperature was 89 degrees, the warmest in Ross-Ade Stadium since a 2011 game against Middle Tennessee State, when the temperature at kickoff was 90. ... The sellout crowd of 60,042 was Purdue's first home sellout since 63,107 -- the pre-renovation capacity -- watched the Boilermakers defeat Indiana on Nov. 22, 2008, in former coach Joe Tiller's final game at Purdue. ... Boilermakers S Jacob Thieneman was ejected for targeting with 4:38 remaining in the third quarter, driving his shoulder pads into the helmet of Michigan receiver Nick Eubanks. ... Purdue LB J'Whaun Bentley was ejected for targeting with 11:20 left in the fourth quarter ... Michigan has allowed only 54 points in 16 quarters.