(AP) - The defending national champions are still an enigma to many, including themselves. Although second-ranked Ohio State is riding a 23-game win streak, the Buckeyes have yet to face a truly significant challenge in 2015.

It finally comes Saturday against No. 9 Michigan State in Columbus.

Ohio State has looked unimpressive at times against unranked teams while repeating the mantra that playing its best football is most important at the end of the season.

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That moment is here for the Buckeyes (10-0, 6-0 Big Ten), who follow this game against the Spartans (9-1, 5-1) with their rivalry showdown at No. 14 Michigan on Nov. 28. Ohio State is No. 3 in the College Football Playoff rankings, leaving little room for error.

"We haven't really been tested," left tackle Taylor Decker said. "This is a huge gauge for us going into some of the biggest games at the end of the season, because this is the biggest game we've played yet this season.

"We haven't even played our best yet," Decker added. "We have to focus on that and not what people are talking about."

These games against their Michigan rivals - tied for second in the East Division - have long been targeted as their most important. If the Buckeyes reach the Big Ten championship game, they could face sixth-ranked and fellow unbeaten Iowa.

"It's do or die time, make it or break it these last few games if we want to get what we trained for," right guard Pat Elflein said. "It's part of being a Buckeye. You feel the pressure all the time to be the best at everything you do."

Ohio State has been struggling offensively beyond running back Ezekiel Elliott, a Heisman Trophy candidate with 15 straight games of 100-plus yards rushing.

The quarterback debate seems over with J.T. Barrett reclaiming the starting job over Cardale Jones after a one-game suspension, but a 28-3 win last weekend at Illinois did nothing to dispel the perception that the offense is not clicking.

Elliott ran for 181 yards and two touchdowns and Barrett chipped in with 150 yards passing and 74 running, but the Buckeyes took a while to get going and had a 14-3 lead at halftime. The run defense was key, holding the Illini to 20 yards on the ground.

"We know there are issues," coach Urban Meyer said. "This is not a perfect team. I've never seen a perfect team. As a matter of fact, we've got a long way to go. There are certain areas we're not playing very good, and that's why we practice all the time."

Ohio State has won 30 straight regular-season conference games since Meyer became coach in 2012, but the Spartans handed him his only Big Ten loss with a 34-24 victory in the 2013 conference title game.

Last season, the Buckeyes went to East Lansing and beat the Spartans 49-37 in an offensive performance Meyer called the best of the season.

"If we perform well (Saturday), it will show everybody we're for real because people are questioning how good we really are," Decker said.

The Buckeyes could certainly be catching a break as well after Spartans quarterback Connor Cook hurt his shoulder in last Saturday's 24-7 home win over Maryland.

Cook, however, insists his shoulder won't be a problem, and the Spartans will surely need a strong performance from him to pull off an upset at Ohio Stadium - although they won their last visit there, 10-7 on Oct. 2, 2011.

Cook has thrown 21 touchdowns and only four interceptions, but he was sidelined in the second half against the Terps after being driven into the ground in the first quarter. Coach Mark Dantonio said afterward that Cook would have played had the game been closer.

"My shoulder is good," Cook said Tuesday. "I'm going to be ready to go so there is no issue with that."

After being ranked No. 2 behind Ohio State in September, Michigan State has had its own issues, including a loss at Nebraska two weekends ago. The Spartans needed a last-second fumble return to stun Michigan and have looked far from dominant.

Through it all, they still had Cook, who threw for 300 yards in four straight games before last Saturday. He's kept drives alive with precise passes on third down and has helped Michigan State remain in the playoff chase.

"He's one of the best quarterbacks we'll face this year," Buckeyes cornerback Eli Apple said.

Cook threw for 304 yards and three touchdowns in the 2013 conference title game and passed for 358 and two scores in last season's loss in East Lansing.

"I think this is another opportunity to establish his legacy. I think he will play well," Dantonio said. "He's always played well on a big stage."