It was bad news on the injury front for fullback David Johnson, the Steelers' newly converted tight end, but the news wasn’t all that bad for rookie OT Mike Adams, the team’s second-round pick.

Coach Mike Tomlin confirmed a season-ending knee injury to Johnson but considered Adams day-to-day with a minor knee injury.

“I don’t want to frame it at this point,” Tomlin said of Adams’ injury after Saturday’s practice. “When he’s ready to go, we’ll put him back out there.”

Adams got his first NFL start during the Steelers’ preseason opener Thursday at Philadelphia and allowed 2.5 sacks on 18 plays.

“I know the mistakes I made were all on me,” Adams said. “Physically, I put myself in some bad situations.

“There are things I can fix easily, so I just have to attack them in practice this week.”

Both injuries happened during the first quarter of Thursday’s game. Johnson, who was on crutches and wearing a brace on his right knee Saturday, left Thursday’s game on the back of a cart after OT Marcus Gilbert rolled on his knee.

“David Johnson’s knee was significant,” Tomlin said. “He has a [torn] ACL, surgery pending.”

Johnson, who caught two passes for 13 yards Thursday, made the offseason switch from tight end to fullback. It was new offensive coordinator Todd Haley’s way to add a physical presence to the run game, lacking since the 2007 season, the last time the Steelers’ featured a permanent backfield blocker.

Prior to the injury, Johnson turned heads in training camp with his stout run-blocking ability and pass-catching prowess from his time as a tight end.

“It was tough,” Johnson said of the injury. “I was down and out about it, but I’m a mentally strong guy. I’ll be alright.”

Will Johnson, a free agent signing from West Virginia -- and the only other fullback listed on the roster -- saw time in the backfield with the first team Saturday. TE Jamie McCoy has also been working at fullback.

“I like to say everything happens for a reason,” said Will Johnson, who played tight end at West Virginia. “I’m not necessarily too happy (David Johnson) had to go down this way, but I’m definitely excited for the opportunity.”

Running backs Jonathan Dwyer (shoulder) and Isaac Redman (groin) also missed Saturday’s practice. Tomlin said the five players on the physically unable to perform list (LB James Harrison, RB Rashard Mendenhall, NT Casey Hampton, OT Max Starks, LB Jason Worilds) will not practice this week.

“We’re assessing those guys,” Tomlin said. “We’ll evaluate them when we come back from the next performance.”

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