Two players who started a combined 30 games for the Cowboys last season might both be done for 2018. 

The Cowboys announced on Saturday that center Travis Frederick has been placed on the team's Non-Football Illness list. Frederick started all 16 games for the Cowboys last season, but hasn't played a single down in 2018 due to a rare autoimmune disease that he was diagnosed with during the preseason. The Cowboys announced in August that Frederick would be out indefinitely after doctors found that he had Guillain-Barré syndrome

Frederick told in September that he has no idea when he might be able to return to football. 

"It's a matter of time for that to happen, and unfortunately, I don't know what that time is," Frederick said. "I think as long as I continue to keep my eyes forward and continue to work toward that, we're going to be in good shape."

The good news is that Frederick's season isn't necessarily over. If the offensive lineman recovers, he'll be eligible to move to the active roster after eight weeks on the NFI list, which means he could conceivably return in December. 

Another player who could return in December is Terrance Williams

The highest-paid receiver on the team, who's pulling in $4.75 million this season, won't be playing with the Cowboys anytime soon, and that's because he's been placed on injured reserve in a somewhat surprising move.  

As the Cowboys' top returning receiver from 2017, there were some high expectations for Williams in 2018, but due to multiple issues, he hasn't lived up to the hype. For one, Williams has been dealing with an ongoing injury since undergoing surgery on a fractured right foot in February. The receiver is also dealing with the fallout of an offseason arrest that is expected to lead to a multi-game suspension

Cowboys executive vice president Stephen Jones also said that Williams has been dealing with some personal issues, which caused him to miss some practice time last week. Williams wasn't active for the Cowboys' 26-24 win over the Lions in Week 4, which marked the first time in his career that he wasn't active for a game. 

"With him, there's a suspension looming out there that might or might not happen," Jones said Friday on 105.3 The Fan, via ProFootballTalk. "He does have some things after coming off that foot injury, which happens. When you have a foot injury, sometimes you rush it back, and you're not quite there. And then just personal and general, some things that maybe are holding him back right now. I just think it's a work in progress with him. He's certainly made a lot of plays for us in the past. We think a lot of him. He's a fighter and competitor, we just got to get him right."

Like Frederick, Williams is eligible to return to the team in December.

The moves by the Cowboys on Saturday opened up two roster spots, and Dallas used one of those immediately to activate David Irving. The Cowboys defensive lineman was eligible to return to the team this week after sitting out the first month of the season due to a four-game suspension for violating the league's policy performance-enhancing drugs.