Nobody is allowing fewer points per game than Michigan and Northwestern.

Coach Jim Harbaugh's unit has posted back-to-back shutouts, though it will take the field for the first time without injured defensive end Mario Ojemudia.

Points are expected to be at a premium in Saturday's Big Ten matchup in Ann Arbor as the No. 18 Wolverines look to stop the 13th-ranked Wildcats from starting 6-0 for the first time in 53 years.

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Northwestern (5-0, 1-0) is allowing 7.0 points per game and Michigan 7.6 for the nation's top marks. The Wolverines (4-1, 1-0) are holding opponents to an FBS-best 19.4 percent on third downs with the Wildcats second at 20.0.

Harbaugh isn't buying into the hype of who has the best defense.

''The two defenses won't go against each other in the game, they won't face other,'' he said.

His squad will be without Ojemudia, a senior who suffered a season-ending Achilles tendon injury in last Saturday's 28-0 win at Maryland. Royce Jenkins-Stone, a senior who has started twice in his career, will take start in his place.

Ojemudia started the first five games this season and made at least one tackle for a loss each time, ranking second on the team with six.

''It's hard to replace a guy like Mario,'' linebacker Joe Bolden said. ''But the greatest thing about football, you've got somebody working just as hard, pushing right behind you, trying to take your spot. We'll get somebody in there to get the job done.''

Michigan has posted consecutive shutouts for the first time since 2000 and hasn't had three in a row since 1980. The Wolverines, winners of four straight, have yielded seven points in their last 15 quarters.

Northwestern has also posted two shutouts this year, winning 27-0 at home over Minnesota in last Saturday's conference opener. The Wildcats are led by linebacker Anthony Walker, who has a team-high 44 total tackles with 8 1/2 for loss to tie for third in the Big Ten.

"They react, as athletic and fast as anybody we've seen," Harbaugh said. "(Walker) is as good a player as we've faced. So all of those things, really good scheme, they play great team defense, leading the country in points allowed. I think that's the thing, mainly, is that this team reacts and flows to the ball as good as you're going to see in college football."

Linebackers Bolden and Desmond Morgan have combined for 62 total tackles and defensive end Chris Wormley has a team-best seven tackles for loss for Michigan.

Ojemudia's loss will be felt since he had two of the Wolverines six sacks in last year's 10-9 win at Northwestern. The Wildcats scored their lone touchdown with three seconds left and went for a winning two-point conversion that failed.

"We got thumped," coach Pat Fitzgerald said. "It's going to be a huge challenge. Those guys are all back, a couple of guys have graduated but most are all back. We gotta bring our big-boy pants, it's Big Ten football."

Northwestern's Justin Jackson was held to a career-low 35 yards on 17 carries in last year's meeting. He's the Big Ten's third-leading rusher this season with 127.2 yards per contest.

Northwestern is averaging a conference-high 248.8 yards per game on the ground while Michigan is limiting opponents to an average of 71.8 for the best mark in the conference.

The Wildcats are 5-0 for the third time under Fitzgerald and last opened with six victories in 1962.

The coach says that Michigan "will be the best team we've played" and this matchup presents a major challenge for new quarterback Clayton Thorson, who has yet to throw for a touchdown against Northwestern's three major-college opponents this year - Stanford, Duke and Minnesota.

The Wildcats have experience facing Michigan quarterback Jake Rudock, who went 2-0 against them the previous two seasons with Iowa with 408 passing yards and two touchdowns.

"Jake has been in the league for a long time, he's played against us multiple times, he's had success at his previous institution and he has a lot of weapons across the board," Fitzgerald said.

Michigan's De'Veon Smith rushed for 121 yards on 18 attempts last season against Northwestern. Smith is expected to return after missing last week's game with a foot injury.

The Wolverines have won four straight over the Wildcats, who have dropped the last three in the final seconds by falling in overtime in 2012 and in three overtimes in 2013. This is the last chance for Northwestern's seniors to beat Michigan.

"Thinking back, those losses definitely stick out to me over the years so I definitely want to get one this week," senior left guard Matt Frazier said.