The Steelers will arrive at training camp at Saint Vincent College on Friday facing uncertainty at several positions, and chief among them is tight end.

No one within the organization knows when Heath Miller will be prepared to play this season. Miller, who led the Steelers with 71 receptions last season, tore his ACL, MCL and PCL. He is still mending from reconstructive surgery.

If Miller isn't ready for the season opener against Tennessee at Heinz Field on Sept. 8, then either Matt Spaeth — in his second tour of duty with the Steelers — or second-year tight end David Paulson is expected to fill in until Miller recovers.

David Johnson, who suffered a season-ending knee injury in the first quarter of the preseason opener at Philadelphia last season, is another question mark. He began last season as the starting H-back, but offensive coordinator Todd Haley figured he would play a more significant role in the passing game.

With Johnson out and wide receiver Mike Wallace easing his way into a new offense after a contract dispute, Miller became quarterback Ben Roethlisberger's go-to target, particularly during the first half of the season. Ultimately, Miller became the first tight end to lead the team in receptions in 46 years.

The absence of Wallace and Miller leaves an apparent hole in an offense that struggled mightily down the stretch in 2012. The tight ends accounted for 96 of 354 receptions and amassed 11 touchdowns, but were largely ineffective during the team's failed bid to advance to the AFC playoffs.

“We're not out here trying to get ready for a game tomorrow,” Roethlisberger said during the OTAs. “We're just out here learning and getting better.'

So far, Johnson is getting better — albeit slowly. But there remains doubt about how much progress Miller has made the past several months. And Paulson was slowed some during minicamp with a couple of nagging injuries.

“I'm about 85 to 90 percent. It continues to get better and better each day,” Johnson said. “I'm coming out to work in hopes of getting the feel back.