CINCINNATI -- Already battling an ankle injury, Bengals tight end Tyler Eifert suffered a back injury at practice this week and will miss Sunday's game against the Cowboys, a source told ESPN.

Eifert, who was not at practice Wednesday, is officially listed as having a "back/ankle injury" on the team's Wednesday injury report.

The tight end saw a doctor off site on Wednesday, a source told ESPN's Bob Holtzman.

Tyler Eifert has yet to fully participate in Bengals practice this season. Kirby Lee/USA TODAY Sports

Eifert, who had offseason surgery on his left ankle, has never given a timetable for his return. The team had expected him to return this week.

On Monday, Eifert was walking with a limp during the open locker room period, but he walked to the practice fields with the rest of the team about an hour later. However, Monday's practice was closed to the media, and the Bengals were not required to give out an injury report, so there is no indication of how much he participated in the session.

When asked whether Eifert would be back this week during his Wednesday morning news conference, coach Marvin Lewis initially evaded the question.

"Tyler?" he asked, prompting the reporter to give Eifert's last name. When given the answer, Lewis smiled and said, "We'll see," before concluding the news conference.

Eifert returned to practice on Sept. 21 after missing the entire offseason, but he hasn't been a full participant yet.