CHAMPAIGN – A backup quarterback is one of the toughest jobs in the world.

Work hard with little playing time and even fewer chances for improvement during workouts because the starter soaks up most of the snaps. But, no matter what, be ready for your number to be called.

Illini freshman M.J. Rivers didn’t even know he was the second stringer until starter A.J. Bush went down in the first half in the non-conference game against Western Illinois Saturday. When it was done, Rivers made a good first impression by passing for two touchdowns and showing some poise in a 34-14 win over the little brother from Macomb.

It’s unknown if Rivers is the backup or the starter when Illinois (2-0) faces South Florida (2-0) at Soldier Field Saturday (2:30 p.m., BTN). Bush suffered a hamstring injury, and coach Lovie Smith didn’t offer details about Bush’s chances of stepping back under center.

But as a backup quarterback, Rivers impressed the coaching staff with his willingness to learn, his passing ability and a poise a little unusual for most freshmen. Rivers helped himself by being a good backup in the preseason.

“You need to be a student of the game, because the backup quarterback doesn’t always take a lot of reps,’’ Lovie said. “You need to be able to take mental reps. You need to be that glue, help the guy on the field playing. Be that coach. When you name is called, you need to step up to the plate by your preparation. To prepare as a backup is harder to prepare as a starter because of the limited amount of reps.

“There have been backups that have never had to play a lot. But when you play, you need to be able to make plays.’’

When Rivers took over against the Leathernecks, the sluggish Illini trailed 7-0. His team needed a spark. The native of Frisco, Texas, never knew he was the second stringer until Bush suffered the injury. After Rivers and freshman Matt Robinson were both warming up on the sidelines, Rivers eventually earned the call.

“When A.J. came out, they called my name,’’ Rivers said. “I was just telling myself, I’ve got this. I know everything. Keep to my reads. Once I was able to get my adrenalin down. I was able to collect myself.’’

It’s a shock to a freshman’s system to play in his first year on campus.

“When a freshman gets to play, especially a quarterback, that’s tough duty,’’ Lovie said.

Despite the slow starts in both games, there’s no quarterback controversy. Bush is the starter. As for Bush’s injury and likelihood of playing against the Bulls, Lovie wasn’t giving much detail, as usual when it comes to an injury.

“When you can’t finish the game, there’s concern, but he’s making a lot of progress,’’ Lovie said.

Yet Bush and Rivers are the same kind of player. Bush earned the starting spot with his preparation and playmaking ability. Rivers is more of a passer than a runner like Bush.

“In the course of a practice, we always try to cater to the skill set of the quarterback,’’ said Illini offensive coordinator Rod Smith. “Most quarterbacks, even if they’re a little bit different, there are some similarities. Our guys can all throw the football. All of them can move to a certain extent. Some go better than others. Some throw better than others. We always want the ability to do both.’’

From Smith’s perspective, there’s only a small difference between Rivers and Bush, other than the age and the experience level.

“I don’t say it’s a different skill set,’’ Smith said. “Both of them are big guys. Both of them throw the football. M.J. has got a little stronger arm than A.J., and A.J. is probably a little more athletic. A.J. was our quarterback because of what he did throughout fall camp. He was the most consistent moving forward. The great thing is we’re building depth with the other guys, getting depth behind him.’’

Rivers wasn’t spectacular, and the Illini relied on a special team play to convert a blocked punt into a touchdown to essentially take control. The Illini wore down the FCS Leathernecks and gained 238 yards on the ground. Yet Rivers didn’t make any huge blunders and completed a pair of intermediate throws for touchdowns. The 16- and 18-yarders went to freshman Ed Carter, who is out of the year, Lovie announced Monday.

The huddle was impressed with Rivers.

“He was as composed as you be,’’ said Illini lineman Nick Allegretti. “I can’t imagine having to step in like he did. He did a great job with it. He ran the offense well. We trusted him back there. We saw how great an athlete he was (in preseason practice), how good a quarterback he is and how good he can be in the future. The offense had a ton of confidence in him. It’s tough to see A.J. go down, but the transition was easy.’’

Looking for trends? The Illini scored more points in each of the first two games than any game a year ago. At the same time, the Illini are 2-0 for the fifth time in the six seasons, and the recent history tells us it might not mean much.

The offense isn’t exactly reaching high gear yet. The Illini fell behind in the first two games, but Rod Smith can see the potential, he said. Perhaps with some coaching up, the Illini can see it, too.

“They had opportunities to say, Here we go again,’’ Smith said. “They started slow in two games in a row. They see plays are there to be made. It’s not a schematic deal. It’s more like we have to do this right, this technique, throw the ball on time here. When the kids see that, maybe there’s confidence on the inside. Maybe they see they can do this. It can be done.’’

With Rivers, the Illini learned they have a backup quarterback. It’s not known yet if he’s promoted to a starting quarterback against South Florida.