Source: TE Eric Ebron walking at practice, has ankle sprain

— -- ALLEN PARK, Mich. -- Tight end  Eric Ebron walked slowly off the Detroit Lions' practice field Monday morning, head down as he moved toward the weight room, ignoring questions from reporters who were being held at a distance.

Ebron did not practice with the Lions on Monday after being carted off the field Saturday during the team's mock game at Ford Field. At the time, his injury appeared serious.

The good news for Ebron is he was walking -- and doing so without a boot or a bulky brace on his right foot.

While head coach Jim Caldwell refused to discuss the injury Monday, a source told ESPN's Adam Caplan that Ebron is dealing with a "pretty decent [right] ankle sprain."

Caldwell declined to say what happened to Ebron, how he felt to see him walking around or whether the injury would keep Ebron out for long. In the past, Caldwell has said when an injury would keep a guy out for a while.

"I'm not going to discuss anything," Caldwell said. "I'm not going to get into prognosticating. I'm not the doctor or anything. I'm not going to talk about that."

Caldwell would only say "we'll see" when asked if Ebron would practice this week when Detroit and Pittsburgh hold joint practice in Latrobe, Pennsylvania, and whether he would play in Friday's preseason opener at Pittsburgh.

Caldwell also declined to say much about what he thought when Ebron initially went down Saturday.

"One of the things that happens in our game is that you're going to have some injuries," Caldwell said. "Things happen all the time that way, but you hope they don't. But sometimes they do."

With Ebron out, veteran Matthew Mulligan and undrafted rookie Cole Wick took first-team reps. The Lions now have their top three tight ends out with injury. Brandon Pettigrew remains on the physically unable to perform list while recovering from a torn ACL. Tim Wright is out for the season with a torn ACL and then there's Ebron.

Ebron spent Monday on the side trying to tutor Wick, often explaining things to him between reps.

"He's definitely been a great mentor," Wick said. "He's helping me out where I need help on and if I'm taking a step too shallow or something, he's there to tell me. Just really helping me out as far as everything goes, on routes, footwork and blocking and everything like that."

Ebron, who is expected to be the Lions' top tight end this season, was having a strong camp. Earlier this preseason, the No. 10 pick in the 2014 NFL draft called the 2016 season a "make it or break it" season. The Lions have to make a decision on Ebron's fifth-year option during this coming offseason.

Lions general manager Bob Quinn also declined comment on Ebron's injury Monday. Detroit did make a roster move Monday, but it was a back-end move flipping wide receivers instead of picking up a tight end.