PITTSBURGH — The Pittsburgh Steelers spent much of the offseason trying to build an offensive line that could keep franchise quarterback Ben Roethlisberger upright — and healthy — for an entire year.

In one squeamish sequence on Saturday night, the Steelers were forced to move on to Plan B.

Pittsburgh lost rookie guard David DeCastro to a gruesome right knee injury in the first quarter of an otherwise dominant 38-7 preseason win over the Buffalo Bills. The 24th overall pick in April’s NFL Draft was carted off the field with what coach Mike Tomlin termed a “potentially severe” injury.

DeCastro underwent an MRI yesterday and while the team declined comment, it was obvious to his teammates he will be gone for an extended period of time.

“He was going to start for us,” linebacker Lawrence Timmons said. “I don’t know the outcome. It doesn’t sound like it’s good.”

ESPN reported DeCastro dislocated his kneecap and tore his medial collateral ligament but did not tear his anterior cruciate ligament, which would have ended his season.

The Steelers could place DeCastro on injured reserve — shelving him for the year — or see how his knee responds to treatment. Either way, he’ll be out for the foreseeable future.

It’s a major blow to a unit that Pittsburgh viewed as an issue after Roethlisberger hobbled his way through the final month of the 2011 season behind a line that struggled protecting him. The Steelers gave up 42 sacks — tied for ninth-most in the league — and Roethlisberger dealt with a series of health issues to his left leg after spraining his foot in a loss to Houston in early October.

DeCastro and fellow rookie Mike Adams — taken in the second round — were supposed to join All-Pro center Maurkice Pouncey and second-year tackle Marcus Gilbert as the core of one of the youngest and most highly touted offensive lines in the league. Instead, Adams is going through some very public growing pains and DeCastro — an All-American at Stanford while blocking for quarterback Andrew Luck — will spend an indefinite period in street clothes.

Pittsburgh will likely turn to veteran Ramon Foster to fill in at guard. It’s a return to the patchwork effort the Steelers have used in the past with mixed results.

A year ago, Max Starks was signed off the street a month into the season to try and hold things together. He’s at it again this season, signing just before training camp as insurance in case Adams had trouble. Starks, who underwent offseason knee surgery, played well while protecting Roethlisberger’s blind side against the Bills and is optimistic even with DeCastro out the line will be effective.

“We’ve been through this before. It’s not like this is something brand new and you’ve had the same five starters for three years and nobody’s had any reps,” Starks said. “Our group is used to turnover and position flexibility.”

The line will need it with the season opener against Denver two weeks away in a rematch of the Broncos’ overtime upset in the wild-card round of last season’s playoffs.

The Steelers have insisted they’ve moved on, though there appeared to be a bit of a hangover by the starters during the first two preseason games. The offense, in the midst of a sea change under new offensive coordinator Todd Haley, finally got on track against Buffalo even with DeCastro sidelined.

Roethlisberger completed 17 of 24 passes for 169 yards and a touchdown, the majority of the yards coming on a brilliant 98-yard drive in the final 2 minutes of the half. The quarterback missed on just one of eight pass attempts during that drive, which culminated in a 6-yard scoring strike to Antonio Brown.

“I just started calling my own plays,” Roethlisberger. “We went no huddle and up-tempo and we were throwing the ball.”

And throwing it downfield. Pittsburgh spent the first two preseason games working exclusively on the short passing game. Though the Steelers completed 57-yard touchdown passes in both games, they were nothing more than long handoffs that turned into big gains.

A 33-yard pass to running back Jonathan Dwyer kick-started the long drive after a penalty backed the Steelers up to their own 1-yard line. Brown, a third-year wideout, was arguably Roethlisberger’s favorite target at the end of last season and he has thrived in Mike Wallace’s absence.

Wallace, a Pro Bowler last year, is a restricted free agent who has yet to sign his one-year tender. He’s expected to return sometime this week, though he’ll find himself playing catch-up on an offense that showed against the Bills it could move the ball without him.

Still, Pittsburgh is eager to have one of the game’s fastest receivers back in the fold. Though now the goal will be making sure Roethlisberger has enough time in the pocket to chuck it downfield to him. It’s something the Steelers insist they can do no matter who is lining up in front of — and alongside — Roethlisberger.

“This isn’t everything,” said backup quarterback Byron Leftwich, who threw for a pair of scores on Saturday. “We still haven’t game-planned for anybody. We’re just calling more plays than we did the first two games. We gave guys more opportunities to make plays and they did their thing.”