LSU has completed three of its four preseason scrimmages but still has stuff to figure out in the nearly two weeks left before the season opener against BYU in Houston.

Injuries have complicated the composition of the offensive line, many of the decisions yet to be made at the skill positions rest involve backup roles, and there’s still competition throughout the defense.

Star linebacker Arden Key looks like he’ll miss the opener as he continues his comeback from offseason surgery, but his absence should be brief.

Meanwhile a stellar freshman class has thrust itself into several equations as Ed Orgeron prepares for his first full season as head coach.

On Saturday, the Tigers completed their longest and most thorough of the scrimmages that Orgeron likes to call “preseason games.”

Here’s a look at what LSU has solved and what it’s still sorting through as it heads into BYU-specific preparations beginning Tuesday:

What’s been solved

1. This was pretty easy: Junior Derrius Guice is the starting running back and he’ll unquestionably be the focal point of the offense. When Leonard Fournette sprained an ankle in preseason camp last year, missed nearly half the games and was hobbled throughout, Guice stepped forward to become the leading rusher in the SEC.

Fournette is in the NFL, but his absence doesn’t mean the Tigers have any lower expectations for their run game. That’s because Guice averaged 7.6 yards per carry last season and seven of his 18 career touchdowns have come from at least 40 yards out.

Senior Darrel Williams, junior Nick Brossette and true freshman Clyde Edwards-Helaire give LSU quality depth, but Guice is clearly the leader just as Fournette was whenever he was healthy.

Tigers observers are curious to see how new coordinator Matt Canada’s offense differs from that which former coach Les Miles employed. One thing that won’t change, though, is the use of an outstanding halfback.

2. This freshman class already is having an impact: Orgeron has named two true freshmen as starters on defense — safety Grant Delpit, whom Orgeron has compared to former Tigers safety Jamal Adams (the New York Jets’ first-round pick in April), and nickelback Kary Vincent, who plays a position Orgeron says LSU will use about 85 percent of the time.

It’s not totally clear exactly how the two will be used as Delpit has been used at both safety spots, battling Ed Paris at free and John Battle at strong. Vincent is trying to hold off Xavier Lewis at nickelback.

LSU likes to call itself DBU for its track record of producing NFL-caliber defensive backs such as Patrick Peterson and Tyrann Mathieu of the Arizona Cardinals, but it’s not just the DBs who have stood out among the freshmen.

Orgeron called outside linebacker K’Lavon Chaisson “dominant” in a preseason scrimmage and Chaisson is battling redshirt freshman Ray Thornton to replace Key in the starting lineup for the opener. The coach also has praised freshman linebackers Jacob Phillips and Tyler Taylor.

On offense, quarterback Myles Brennan also has gotten some first-team reps (more on Brennan later) and Orgeron has singled out Edwards-Helaire. Additionally, three linemen — guard Ed Ingram and tackles Saahdiq Charles and Austin Deculus — seemed up to the task when working with the first team.

3. O-line taking shape: Speaking of the line, it has endured a series of what Orgeron called “camp injuries” but everyone is back on the field and the unit is pretty well set. K.J. Malone is the left tackle (the only returning starter at the same position), Garrett Brumfield is the left guard, Will Clapp (two-year starter at guard) is the center and Toby Weathersby is the right tackle.

The one position that’s still a competition is right guard, where Ingram is pushing redshirt freshman Lloyd Cushenberry to start at a spot that was held by incumbent Maea Teuhema before he left the program.

Ingram, Charles and Deculus got significant first-team reps while Malone, Clapp and Weathersby were sidelined and each showed well enough for the coaches to trust them if they need to put them in a game.

But, health permitting, LSU should have a solid unit in which the true freshmen are able to bide their time.

What’s left to fix

Though LSU still has some questions to answer, it still has time to do so before kicking things off.

1. Who starts at QB? Fifth-year senior Danny Etling probably will take the first offensive snap against BYU, but it wouldn’t be surprising if others play against the Cougars.

LSU has installed a package to take advantage of the running skills of true freshman Lowell Narcisse, and Brennan has impressed Orgeron during camp. The Tigers are in no hurry to thrust Brennan into the lineup, but they are intrigued by his passing skills. He may well be the quarterback of the future and any time playing time he gets early on would help the coaches determine precisely when that future should begin.

Additionally, sophomore Justin McMillan is in the mix and he has the most experience after Etling.

Quarterback competitions have become featured attractions during LSU preseason camps and even though Etling’s reasonably secure grip on the starter’s position has lessened the drama this year, it doesn’t mean the competition is over.

“(Brennan and Narcisse) are going to be great quarterbacks in the future,” Orgeron told reporters. “If they don’t start, hopefully we can play both of them this year. I don’t know if we can do that. … Both of them can do great things.”

2. D.J. Chark is the No. 1 wide receiver and the rest of the primary targets are …? Well, maybe the rest of preseason camp will yield some sort of pecking order, or maybe the game against BYU will do that.

The bottom line is Chark, who hadn’t caught a pass before grabbing 26 for 466 yards and three touchdowns last season, is by far the most experienced receiver the Tigers have.

Chark also rushed 12 times for 122 yards and two more touchdowns last season and that skill figures to be used even more in Canada’s offense.

Canada threw the ball a little more than a third of the time last season at Pitt, and with Derrius Guice at running back, there’s no reason to expect that percentage to go up.

LSU is hopeful that sophomore Drake Davis will have a breakout season and emerge as a big playmaker as the second receiver and that senior Russell Gage will become a threat from the slot.

There’s a laundry list of unproven others still trying to carve out a role — sophomores Derrick Dillon, Dee Anderson and Stephen Sullivan as well as freshmen Mannie Netherly, Racey McMath and Justin Jefferson. Another freshman — JaCoby Stevens — was moved to wide receiver from safety early in camp to give him a better chance of playing right away.

3. LSU has yet to finalize a few spots on its special teams. The one specialist position that is settled is punter, where incumbent Josh Growden is the guy after putting 22 of his 57 punts inside the 20 as a freshman last season.

Redshirt freshman Connor Culp and sophomore Jack Gonsoulin are competing to be the place-kicker.

Guice handled 11 of 24 kickoff returns last season, but given his importance to the offense and a healthy collection of capable returners, Guice is expected to relinquish that role. The front-runners to handle kickoffs returns are Drake Davis and Edwards-Helaire. Donte Jackson and Brossette have experience returning kicks.

The Tigers need a new punt returner because cornerback Tre’Davious White, who was a first-round draft choice of the Buffalo Bills in April, handled all the punt returns last season. Davis is the front-runner to be the punt returner, though Jackson and Derrick Dillon have gotten some reps.