Grant Cohn: Five 49ers whose roster spots look to be in jeopardy

Most football teams give young players a few years to develop before replacing them with more young players. The 49ers may not be so patient.

With the exception of the current rookies, every young player on the Niners was drafted to fit Jim Harbaugh's system, which later became Jim Tomsula's system, neither of which have anything to do with the current system under Chip Kelly.

Harbaugh and Tomsula coached 'phone-booth football' — 11 players trying to pound the football through the teeth of the defense. Kelly coaches spread football — 11 players trying to force the opposing team to defend the entire width of the field. Polar opposite philosophies.

Harbaugh and Tomsula needed tough players to make their philosophy work. Kelly needs fast players. He'll have tremendous freedom to cut whomever he feels doesn't fit his system.

Here are five big-name young players Kelly might cut before the regular season.

1. WR Quinton Patton, fourth-round pick, 2013.

Kelly typically starts three wide receivers, and Patton currently is one of the starting three. As he should be.

Patton deserves a chance to start. He hasn't gotten many opportunities to play since entering the league, and he'll be a free agent next offseason. He needs to play and play well to get a second contract.

Unfortunately for Patton, he didn't play well during the two OTAs that were open to the media this offseason. Didn't get open or fight to make contested catches. Went through the motions. Niners quarterbacks hardly ever threw to him.

In Patton's defense, during April he had a cast on his right arm, and the arm may not yet be 100 percent. It had better heal soon, though, because his backup, Eric Rogers, was the Niners' most impressive receiver during the first two OTAs. And Rogers' backup, Devon Cajuste, also was impressive before he rolled his ankle — a minor injury.

Patton must outplay Cajuste and Rogers to make the team. The Niners signed those two undrafted free agents this offseason. They're Chip-Kelly guys. Patton is not.

2. RB Mike Davis, fourth-round pick, 2015.

Another player who is not a Chip-Kelly guy. Davis is a Jim Tomsula guy — a tough 'downhill' runner who is not elusive, does not make defenders miss. Davis runs through defenders and picks up yards after contact.

At least that's what he did in college. In the NFL last season, his rookie season, he ran through no one. Picked up just 58 yards on 35 carries. Jarryd Freaking Hayne was better than Davis, and Hayne retired.

Now Davis has to play in an offense that requires elusive running backs who run parallel to the line of scrimmage before making a cut and juking a defender. That's not what Davis does.

Davis also has to compete for a roster spot with one of the most elusive running backs in the draft this year, rookie sixth-round pick Kelvin Taylor, who's a perfect fit for Kelly's offense.

Good luck, Mike.

3. C Marcus Martin, third-round pick, 2014.

The Niners drafted Martin two years ago to back up starting center Daniel Kilgore and eventually take Kilgore's job.

So far, Martin has been an abysmal failure. His first two seasons in the NFL, he had to start 22 games due to injuries along the Niners' offensive line, and he simply wasn't strong enough to play.

This year, Martin has to compete against undrafted free agent Alex Balducci, who certainly is strong enough to play center in the NFL — he just has no experience at that position. He played defensive tackle at Oregon under Kelly.

If Balducci proves he can make the correct calls at the line of scrimmage and snap the ball straight when the quarterback is in the shotgun, he probably will make the team. So far, he seems proficient in both areas.

Martin better be in the weight room right now. He's in trouble.

4. G Brandon Thomas, third-round pick, 2014.

The Niners spent a third-round pick on Thomas a few months after he tore his ACL during a pre-draft workout with the Saints. He didn't play his first two seasons in the league.

This offseason, Thomas has played right guard for Niners' first-team offense. But he won't play that position when the season starts. He merely is keeping the spot warm for rookie first-round pick Joshua Garnett, who hasn't yet graduated from Stanford. He'll graduate later this month and join the team for training camp.

Which means Thomas will compete for a job as a backup offensive lineman. And he'll compete against three players who play guard and tackle — Trent Brown, John Theus and Fahn Cooper. Thomas is too short to play tackle in the NFL. He's a highly specialized backup guard on a team that values versatile bench players.

Bad fit.

5. CB Kenneth Acker, sixth-round pick, 2014.

After spending his rookie year on the Injured Reserve list, Acker started 13 games at cornerback last season and intercepted three passes. He was one of the best young players on the team and a favorite of coach Tomsula and defensive coordinator Eric Mangini.

They seemed to like his toughness. Acker is a good tackler for a cornerback who helps the run defense. But he's terrible in man-to-man coverage. In fact, he's so bad Mangini hardly ever used man-to-man coverage last season. He used zone coverage because that's what Acker could handle.

New defensive coordinator Jim O'Neil rarely uses zone coverage. He hasn't even taught zone-coverage techniques during OTAs. He wants his corners matching up man to man against the best receivers in the league, and Acker can't do that.

Another bad fit. Don't get attached to this guy.

Grant Cohn writes sports columns and the 'Inside the 49ers' blog for The Press Democrat's website. You can reach him at grantcohn@gmail.com.