ALLEN PARK —  Jeff Backus has been such an anchor on the offensive line for the Detroit Lions, it’s hard to imagine anyone else at left tackle.

 No. 76 was always there.

 After missing just one game in 12 seasons, Backus’ retirement was announced on Thursday. 

“It’s something that was not unexpected. We’ve been talking with him a little since the season ended,’’ team president Tom Lewand said. “It was a tough decision for Jeff, but one he’s at peace with.

Fans haven’t always been crazy about Backus, but they will probably miss him when he’s gone. Last season he allowed just one sack according to Pro Football Focus.

So now what for the Lions’ offensive line?

Rob Sims will return at left guard and Dominic Raiola at center. Other than that the line is a bit of a puzzle.

 Along with left tackle, there are openings at right guard and right tackle. Stephen Peterman was released and Gosder Cherilus signed with the Indianapolis Colts as a free agent.

 You can speculate all you want, but my guess is that even the Lions aren’t sure who will play where when the season starts in September.

 Riley Reiff, a first-round pick a year ago, could get the start at left tackle. He was drafted with the idea he’d eventually take over that position when Backus left. The Lions, however, have been careful to speak in general terms about Reiff’s future.

“We’re comfortable with, obviously we drafted Riley Reiff last year, we’ve got Jason Fox and Corey Hilliard,’’ Lewand said. “... We’ve got some good competition there. As Martin (Mayhew) has said there is no finish line when it comes to player acquisition. So we have a lot of opportunities going forward. (We’re) looking to add to the mix to the offensive line both from the outside and inside.’’

Coach Jim Schwartz has said he sees Reiff at tackle rather than guard. But Reiff, who played left tackle at Iowa, could play any of the four spots other than center.

 Reiff filled in when Backus couldn’t finish a game last season and then started at left tackle in the one game that Backus missed (in 12 seasons). He looked like a rookie early in that game, but then held his own.

 Reiff should be fine eventually. He got some playing time last season, also lining up on the outside of the right or left tackles. There’s a reason he was a first-round pick, he just needs more snaps.

 Fox, 24, and Hilliard, 27, could compete for the right tackle position.

 Hilliard saw spot duty in 2011 for Cherilus and performed well. He didn’t play at all in 2012, but the Lions re-signed him last week so you know they like him.

 Expect Bill Nagy and Rodney Austin to compete for the right guard start.

 Nagy, 25, was picked up by the Lions after being waived by the Dallas Cowboys last season.

 The former Wisconsin lineman can play guard and center. He started four games at guard for the Cowboys in 2011 and was competing for the starting center spot before he injured his ankle.

 Austin, 24, impressed Lions’ coaches while on the practice squad last season. In fact, he was so impressive Mayhew said they were nervous another team might pluck him off the practice squad. 

 “He’s got an opportunity to be a factor this year,’’ Mayhew said last month.

 The Lions knew Backus’ departure was a possibility and it didn’t play a factor in deciding not to re-sign Cherilus. He was a first-round Lions’ pick, but not Mayhew’s pick.

 Backus had a year remaining on his contract so his departure will open up $3.5 million in salary cap space.

 It’s just too early to know what the offensive line will look like in 2013. Free agency isn’t over and the draft is more than a month away.

 Offensive tackles Luke Joeckel (Texas A&M) and Eric Fisher (Central Michigan) are expected to be drafted in the top five. The Lions have the No. 5 pick. 

  But Backus’ departure does not necessarily mean that either one will be targeted by the Lions who have more needs on defense.

 Give it time. Free agency is not over. Trades are also a possibility before the draft.

 Even then, once camp starts we could see musical chairs from day to day to figure how which player excels at which position.

 It’s early. Re-tooling the offensive line is a process that will take time.

  (Follow @PaulaPasche on Twitter. Order her new book,  “100 Things Lions Fans Should Know & Do Before They Die’’ here. It’s also available at bookstores.)