EARTH CITY, Mo. -- With no shortage of tall, athletic receivers on the Chicago Bears' roster, the challenge for the St. Louis Rams secondary this week will be to find ways to limit the amount of alley-oops the Bears convert.

The onus will likely fall to Rams cornerbacks Janoris Jenkins and Trumaine Johnson, both of whom are well aware of how hard it can be to handle the likes of Chicago’s Brandon Marshall and Alshon Jeffery when the ball goes up.

[+] EnlargeAP Photo/G. Newman LowranceRams cornerback Trumaine Johnson has been getting a lot more playing time with starter Cortland Finnegan banged up.“You have got to finish strong at the top of the route,” Johnson said. “Those guys are basketball players, when they throw the ball up there they go get it, so you have just got to be strong at the top of the route and try to get the ball out.”

For Johnson, staying strong and physical during all portions of the route has been a work in progress that is starting to yield some good results.

With normal starting cornerback Cortland Finnegan battling injuries for most of the season, Johnson has been thrust into a role far more involved than his previous job as the third corner.

Johnson has handled the starting job opposite Jenkins in five of the six games since Finnegan suffered a leg injury against San Francisco in Week 4. Aside from a hiatus against Tennessee when Johnson played just 21 snaps while Finnegan and Jenkins played on the outside, he’s averaged 58 snaps in the five other contests.

Along the way, Johnson has seemingly made strides in every week. For the season, he’s racked up 39 tackles, two interceptions and four pass breakups.

“Really well,” defensive coordinator Tim Walton said. “He’s coming along really well, and that’s the thing with him. You like to see the growth of the guys, and you can see each week that fundamentally he’s getting better, understanding the game, being more aggressive in the man and zone coverages. I’m pleasantly surprised with his progressions.”

Walton’s surprise with Johnson’s improvement has led to a bit of a philosophical shift in terms of defensive approach. Although the Rams still make defensive calls based on situations, they’ve been more aggressive overall, in no small part because the coaching staff has more faith in Johnson to play press coverage near the line of scrimmage.

Johnson said that has been an adjustment he’s had to make since arriving in the NFL after playing man about half the time during his college years at Montana.

At 6-foot-2, 204 pounds, Johnson’s size would seem to make him an ideal candidate to play a more aggressive, physical style. But before he could make that transition, he said he had to find his place in the defense.

“This is my second year here. I am feeling more comfortable every day, to be honest with you,” Johnson said. “They stress, with Tim coming here, guys playing one on one and pressing, so that’s what I’m working on.”

The challenge against Chicago will likely be the most difficult yet for Johnson and Co. The combination of Marshall (6-4, 230) and Jeffery (6-3, 216) is one of the largest in the league.

Bears quarterback Josh McCown will not hesitate to throw the ball up and simply let his oversized receivers go up and get it against smaller defensive backs. Jenkins has had his ups and downs against bigger wideouts this season, but Sunday’s game could come down to how well Johnson fares against receivers he’s built to cover.

“Timing is a big thing, and try to get hands on them at the line the whole game,” Johnson said. “We’ve got our hands full.”