In what could eventually fuel improved pass protection, Long took reps on the outside after earning Pro Bowl bids in his first two seasons at right guard.
"We've got a couple of guys on the shelf right now and it's something we wanted to take a look at," Fox said Wednesday, via the Chicago Tribune.
As NFL Media's Daniel Jeremiah points out, some people in the league believe Long will end up starting at right tackle in 2015.
"I was a little like a fish out of water, but it's to be expected," Long said. "I've always thought I could do it, but who knows? We have guys who play right tackle."
Pro Football Focus graded Mills as their 66th rated offensive tackle last season out of 84 players.
A potential shuffling of the offensive line this offseason under new coordinator Adam Gase is one of the many changes we'll track in Chicago as the summer presses toward training camp and preseason games.