The 29-year-old Thomas had been very vocal about his desire for a new deal as he enters the final season of a four-year, $40 million contract and is set to make $8.5 million in 2018.
Elliott said he needs to have "better awareness" of where he is on the field and felt the fumble "cost us the game".
Seattle Seahawks free safety Earl Thomas did not participate in Friday's practice for personal reasons, according to coach Pete Carroll. It ended with Thomas telling reporters that he doesn't plan to practice for the Seahawks if he doesn't feel like it.
Thomas' absence earlier this week comes after he was a holdout for the offseason and preseason.
"I'm investing in myself", the ninth-year veteran added.More news: Gronk nearly dealt to Lions at draft
While there will no doubt be rumors that Thomas is not playing to avoid getting injured, which would prevent him from getting traded to Dallas, it is entirely possible Thomas sat on Friday for other reasons.
"I had no idea [I was out of bounds]", Elliott said.
ESPN's Chris Mortensen also reported that the Chiefs "have emerged as a candidate if a deal is made before the October 30 trade deadline".
It was a awful day for the Cowboys' starting running back. "Me being a leader on the team, me being a better player on this team, I got to do a better job of taking care of the ball".