LAKE FOREST, Ill. -- Mike Glennon is the clear front-runner to start at quarterback for the Chicago Bears on Sunday in Tennessee, but rookie Mitchell Trubisky deserves the opportunity to play a couple of series with the first team.

Those screaming for the Bears to pull the plug on Glennon after two iffy preseason performances are likely to be disappointed. Chicago has given no indication -- yet -- that Glennon’s no longer the guy. To date, Glennon has taken virtually every quarterback rep with the No. 1 offense since he signed here in March.

If the plan is for Glennon to start Week 1 of the regular season against the Atlanta Falcons, then it makes perfect sense for him to start the all-important third preseason game versus the Titans.

How much can Mitchell Trubisky give the Bears this season? Now is the time to find out. Photo by Jonathan Daniel/Getty Images

In case you haven’t noticed, Glennon (two preseason interceptions) needs the work.

But let’s not be naïve about Glennon’s long-term future with the Bears.

There is none.

Trubisky, the second overall pick, eventually will be Chicago’s starting quarterback. Right now, the Bears have to gauge Trubisky’s readiness, or lack thereof, to take over the reins.

What if Glennon goes down with an injury? Or what if Glennon plays poorly enough in the first quarter of the regular season to justify a change?

You can’t get a true evaluation of Trubisky watching him compete exclusively against second- and third-stringers. He needs a little bit of live action alongside the starters -- and against starting-caliber defensive players -- and Sunday is the absolute last chance for that to happen before the games count for real.

Let’s say Trubisky struggles in limited action versus the Titans’ No. 1 defense.

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Well, that’s why the Bears signed Mark Sanchez, who hasn’t played much in the preseason because Chicago’s coaching staff knows exactly who he is.

But if Trubisky does well in Nashville, and continues to improve and develop on the practice field, the Bears know in the back of their mind that they can turn to the rookie -- when the time is right.

Many argue the time to make the permanent switch to Trubisky is now. But by Trubisky’s own admission, he’s still learning how the game works at the NFL level. You can’t start a quarterback who’s unable to identify the Mike linebacker on defense. The good news is Trubisky comes across smart enough to master all these quarterback nuances, but it takes time -- especially for a player with only 13 career collegiate starts.

The first step, though, is getting Trubisky on the field with the starters this weekend -- even if it’s in the two-minute drill at the end of the first half.

Glennon had two preseason games and three-plus weeks of practice to quiet any talk of a quarterback controversy with Trubisky. Unfortunately, Glennon didn’t play well enough for that to happen.

Now it’s time for the Bears to get a better idea of what they have in Trubisky.