Pittsburgh Steelers first-round draft choice David DeCastro, shouldered with Alan Faneca’s No. 66 and similar expectations at guard, lasted six plays in his inaugural professional preseason start Saturday. The way he left, it didn’t look good.

DeCastro was carted off the field with a right knee injury, the extent of which wasn’t immediately divulged.

“It’s potentially severe,” Mike Tomlin said after the Steelers’ 38-7 romp at Buffalo. “But we’ll see what the MRI says.”

The way DeCastro’s right knee was bent back on pass protection in the end zone on third down, a hyperextended knee might be considered a lucky result. Surgery, though, wouldn’t be a surprise.

Whatever happens, count on Ramon Foster starting at right guard Thursday in the final preseason game against Carolina at Heinz Field. Foster has been a hard fellow to keep out of the Steelers’ starting lineup in his three previous seasons, including 22 of 27 games in which he played in 2010 and 2011.

Remember, this is a bedraggled Steelers lineup already missing RB Rashard Mendenhall (ACL), WR Mike Wallace (contract), TE David Johnson (ACL, out for season), NT Casey Hampton (ACL/elbow), OLB James Harrison (knee), OLB Jason Worilds (wrist) and ILB Stevenson Sylvester (knee). Only Mendenhall and Hampton are practicing heading into training camp’s final week. DE Brett Keisel’s high-ankle sprain in Saturday’s first half was deemed “mild” by Tomlin, who likely won’t play Keisel on Thursday.

There is a fullback after all: Keep an eye on street free-agent Will Johnson, who sat out 2011 after finishing his college career at West Virginia (where he moved from TE). He played most of the first-team Steelers snaps through two quarters Saturday, including Isaac Redman’s TD run over the left side where Johnson blocked. He continued to fare pretty well as a blocker, and the coaches’ confidence in him seems to indicate that he’ll not only make the roster but could receive fullback playing time.

CB or not CB: Cortez Allen opened the game as the nickel back, then made some nifty pass breakups in a regular corner role. Undoubtedly, he has secured his place at the nickel. But if Keenan Lewis falters, Allen -- in his second year -- has shown the ability to start, and coaches have shown faith in him.

Special teams gets new assistant coach: Assistant Amos Jones replaced the abruptly fired Al Everest on Thursday. Tomlin announced postgame Saturday that he was replacing Jones with himself. “Nothing that we can’t handle,” Tomlin said, punctuating it with a smile. “I look forward to that challenge, moving forward with myself and Amos Jones. I’m going to help a little bit more. I’m looking forward to doing that.”

Getting to 53: The Steelers must pare the roster to 75 by Monday, and the Buffalo game went a ways to settling the eventual regular-season roster.  Even with the expected return of Wallace, there remains a battle for No. 5 WR -- David Gilreath was entrenched after last week, but Derrick Williams made his bid with a TD and two catches for 36 yards Saturday. Robert Golden continued his solid second-half play and bid to be a reserve CB, returning a Vince Young pass 47 yards to set up Williams’ TD.

Follow Chuck Finder on Twitter for real-time updates from Steelers training camp @CBSNFLPit.