Only four training camp practices in, and the Green Bay Packers may already need to replace a 12-game starter who played over 500 snaps on defense last season.

The Packers fear linebacker Jake Ryan suffered a season-ending knee injury during Monday’s practice.

Here are a few ways the Packers could replace Ryan in 2018:

Rookie LB Oren Burks

The next man up is the most obvious solution. The Packers took Burks, a converted safety from Vanderbilt, in the third round of the 2018 draft. It’s possible he’ll slide in next to Blake Martinez and give the Packers defense an athletic option for covering tight ends and running backs. Can he hold up against the run? There will be growing pains if Burks has to play extensively as a rookie. Rookie Greer Martini might get a look, too. The Notre Dame product has been running with the second-team defense at inside linebacker.

Sign a free agent

The Packers are so thin behind Ryan and Martinez that new GM Brian Gutekunst might have to dip back into the free agent market and find a veteran. Burks and undrafted free agents Greer Martini, Marcus Porter and Naashon Hughes are rookies, and Ahmad Thomas is another converted safety who has never played an NFL snap. Unless Burks blows up the rest of camp, it would be very difficult for the Packers to go into 2018 without adding some proven help. Veterans NaVorro Bowman and Brian Cushing are among the names still available.

More Clay Matthews at inside linebacker

The Packers were already planning to give Matthews some snaps at inside linebacker, so it’s possible Mike Pettine and Mike McCarthy will simply ask him to do more inside after losing Ryan. It would be a risky strategy – the Packers are equally thin on the edge – but Matthews is a proven commodity inside. He was excellent there for stretches in the past. In fact, his current skill set might be perfect for playing inside linebacker. It’d likely take a big camp from one of the young edge rushers – such as Kyler Fackrell, Vince Biegel or Reggie Gilbert – for the Packers to seriously consider a more full-time switch for Matthews inside.

More Josh Jones at inside linebacker

This would be an attractive option if Kentrell Brice hadn’t exited Monday’s practice on a cart. Jones played a bunch of snaps at inside linebacker in 2017 and thrived early in the season. The best game of his rookie season – a win over the Bengals at Lambeau Field in Week 3 – came when the Packers asked him to do almost everything at inside linebacker. Maybe the Packers are comfortable with a guy like Marwin Evans or Jermaine Whitehead (or even Quinten Rollins?) taking over snaps at safety, allowing Jones to slide into the box as a linebacker. Adding a free agent at safety like Eric Reid or Kenny Vacarro would serve the same purpose. The Packers have also tinkered with Rollins and Whitehead playing inside linebacker in subpackages early at camp, although neither would be expected to replace Ryan in the traditional 3-4.

Schematic changes

New Packers defensive coordinator Mike Pettine could consider scrapping most of the defense’s 3-4 looks on early downs and obvious run situations and play a more traditional four-man front. The Packers have the horses along the defensive line, Nick Perry is capable of playing on the end and Clay Matthews has roving ability as an off-the-ball linebacker. Luckily, the Packers defense plays far more snaps in the nickel than the traditional 3-4, so losing one starter at inside linebacker isn’t a scheme-shattering loss, even though Ryan has developed into a strong run stuffer. A few tweaks here and there could soften the blow.