Brian Kelly was asked Sunday about the growth of his defense since Notre Dame's loss last season to Navy, which utilized the triple-option offense to run for four touchdowns and 367 yards -- including 210 from Alexander Teich -- in a 35-17 rout by the Midshipmen.

"You know, I think the Navy game was schematic more than it was kids not understanding how to play the right kind of defense," Kelly said. "So if you look at that as not about our players, I think we've showed steady improvement since last year. I think it's continuously been better and better defensive play leading up to where we are right now.

"We still have a ways to go. But I will tell you this, that it's tough to run the ball on us, and that's where you wanna start. So our starting point was, be difficult to run the ball on, and then let's continue to improve in the back end."

After that loss last Oct. 23, the Fighting Irish did not allow another 100-yard rusher over their last five games and gave up just one touchdown run, a 1-yard sneak by USC quarterback Mitch Mustain. Over their last four games, Notre Dame did not allow so much as a 50-yard rusher.

Only one player has rushed for more than 100 yards against the Irish since their loss to Navy, and that was Denard Robinson in Week 2 of this season. Robinson's 16-carry, 108-yard effort on the ground Sept. 10 is just as notable for another number Shoelace put up that night: One, as in the number of rushing touchdowns he scored. As in the only rushing touchdown Notre Dame has allowed this season.

And even that could be considered a fluke, as Robinson simply scooped up the ball and took it in for a 1-yard score after Irish safety Harrison Smith jarred it from Michigan running back Stephen Hopkins.

In allowing just one score on the ground this year, Notre Dame is tied for the nation lead with four other schools. The Irish have allowed just 91.2 yards per game on the ground this season, good for 19th nationally.

All of which is a roundabout way of saying that this Saturday's contest against Air Force and its triple-option offense should be a fascinating test of execution on each side of the ball.

The Falcons' 364.5 rushing yards per game are good for third in the nation. Only seven teams have scored more than Air Force's 15 rushing touchdowns, but six of them have played five games to the Falcons' four.

The challenge could be all the more difficult if defensive end Ethan Johnson cannot play. Johnson, a two-year starter, suffered a right ankle sprain in Saturday's 38-10 win at Purdue, and Kelly would only say that there's a chance he could return this weekend.

"We'll immobilize him for the next few days and then get him moving and see," the second-year coach said. "It's one of those things where it's such an individual case-by-case situation when it comes to ankles, so he'll be immobilized.

"Last night he was in a boot. He'll stay in that until probably mid-week and then we'll start moving him and see what he looks like."

If Johnson can't go, Aaron Lynch will likely get the start. Known for his ability to rush the passer, Lynch saw more action Saturday after fellow freshman Stephon Tuitt did not make the trip to West Lafayette, Ind., because of a violation of Kelly's missed-class policy.

Kelly said after the game he expected Tuitt back, but neither Tuitt nor Lynch played Week 2 at Michigan because of the complex offense the Wolverines and, more specifically, Robinson, ran.

In any event, it makes this week's contest all the more important, one that can't be overlooked with a bye week and USC looming afterward.