Johnson may have gained the edge Sunday in an evening practice at Rams Park. Gaines left practice early with a foot injury. He walked off on his own and did not return.
St. Louis coach Jeff Fisher said he was unsure of the full extent of Gaines' injury.
''Don't know. Going to have to do some tests,'' Fisher said. ''He got stepped on, which is encouraging as opposed to the other things, so we'll do some tests. We'll find out.
''Right now, they're not too concerned; he's very sore, but that typically can happen when you get stepped on.''
Even before the injury, Fisher said he believed Johnson was ahead in the competition for the starting position opposite Janoris Jenkins. It's one of the few positions on the Rams that's being contested in camp.
''Yeah, EJ is a little bit behind right now,'' Fisher said. ''Trumaine is not behind. Trumaine's in as good of condition as we've seen since he's been here. It'll be great competition.
''It's a win-win for us. We've got a rookie from last year that played really solid defense for us. We've got a starter that unfortunately suffered a significant injury in the preseason and really never made it back other than some sub stuff toward the end. Both of them have the flexibility to play inside, play outside so it's a good position to be in.''
Johnson, a third-round pick in 2012, suffered a knee injury in the third preseason game against Cleveland. He missed the first eight games in 2014. The injury is in the past for him.
''Man, I'm healthy,'' Johnson said. ''I was completely healthy four months ago. There's no tape or anything on my knee. Everything is solid.
''Injuries are part of the game. You have to be mentally strong to rehab and get back out here. I like to be out there with my brothers. It was hard to sit and watch last year.''
Gaines, a sixth-round pick from Missouri, stepped in when Johnson went down. He did not play like a rookie. Gaines started 15 games before a concussion cost him the final game last season.
Gaines finished with 70 tackles, two fumble recoveries and two interceptions.
Johnson is set to be a free agent after the year and Gaines is under contract for three more years.
''This is my contract year, but my main focus is winning games here,'' Johnson said.
Fisher said he is not leaning one way or the other.
''We're going to just kind of alternate them every day and see what happens,'' Fisher said.
That's fine by Johnson. He welcomes the challenge of winning the job.
''You've got to love competition man,'' Johnson said. ''Me and him are competing right now. We're making each other better.''
Along with Jenkins, the Rams also have more depth with Lamarcus Joyner, a second-round pick in 2014.
Fisher is encouraged by what he has seen from Joyner in camp.
''Lamarcus has come on as well, not only outside, he can play inside and on the slot and the nickel,'' Fisher said. ''You know in Lamarcus' situation, what a difference a year makes for a young player to come back in year two and finally just get it. He gets it now. He'll be productive for us.''
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