ARLINGTON, Texas (AP) — Dallas Cowboys center Travis Frederick said Wednesday he has been diagnosed with a rare neurological disorder that causes weakness in various parts of the body, and the four-time Pro Bowl player isn't sure on a timetable for a return.

Frederick said he has received two treatments for Guillain-Barre Syndrome over the past 48 hours and that the treatments will continue for several days.

"I am very optimistic about my condition and the immediate future, as I have been told that the illness was detected at a fairly early stage," Frederick said. "My doctors have told me that it is not possible to determine a timetable for a return to the field right now, but I am hopeful that I will be able to play as soon as possible."

The 27-year-old Frederick has started all 83 games, including three playoff games, since the Cowboys drafted him late in the first round in 2013. That streak is likely in jeopardy with the opener 2 1/2 weeks away, Sept. 9 at Carolina.

According to the National Institute for Neurological Disorders and Stroke , Guillain-Barre causes the body to attack a network of nerves around the brain and spinal cord. Most people recover from even the most severe cases of the auto-immune disease, but some will continue to have some degree of weakness, according to the institute.

Frederick saw a specialist in California while the Cowboys were still in training camp there because of feelings he said were similar to stingers in his neck and shoulders. When he returned to camp, he told reporters he checked out OK. He was on the sideline for the preseason game against Cincinnati on Saturday.

Coach Jason Garrett said Tuesday tests for Frederick continued with doctors in the Dallas area. Garrett declined to be more specific.

Joe Looney has been working at center in Frederick's absence, but the Cowboys have always considered 2014 All-Pro right guard Zack Martin a possibility there. Martin will miss the rest of the preseason after hyperextending his left knee against the Bengals.

Frederick was the second of three first-round picks in a span of four years, a group that anchors what is considered one of the NFL's best offensive lines. Left tackle Tyron Smith, the 2011 choice, has battled back issues in recent years. Frederick, Smith and 2014 pick Martin have all been named to the Pro Bowl each of the past four seasons.

Dallas is banking on a healthy offensive line in front of 2016 NFL rushing leader Ezekiel Elliott, with the Cowboys hopeful that quarterback Dak Prescott and a largely unproven group of receivers can feed off a powerful running game.

Two years ago, Frederick signed a $56 million, six-year extension that at the time made him the highest-paid center in the NFL. He's now fourth on that list.

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