PITTSBURGH — Pittsburgh Steelers first-round pick Derrick Harmon said Saturday he initially feared the MCL sprain that took him out of the preseason finale was “serious.”

“Most definitely,” Harmon said, asked if he was relieved at the diagnosis. “I thought it was serious, but it wasn’t as serious. But still something [to] get rehab on and that’s what my main focus is right now, is just to attack rehab and do what I’ve got to to get back on the field and help the team.”

Harmon, selected No. 21 overall in April, echoed coach Mike Tomlin’s earlier timeline that the defensive tackle is “week-to-week.” Depending on the degree of severity, MCL sprains can take from several weeks to a month to heal. With the season opener still two weeks away, Harmon could avoid landing on short-term injured reserve.

Harmon, who added he doesn’t need surgery, said the injury happened when he was hit in the knee in a play before he left the field. He said he tried to play through it but decided he needed to get it checked out. That’s when he exited to the locker room on a cart with a towel over his head and a forlorn look on his face.

“I didn’t know what it was at the time, that’s why I was so in that mood, in that head space,” he said.

Harmon returned to the sideline by the fourth quarter and was laughing alongside teammates Cameron Heyward and T.J. Watt on the bench.

“Really just don’t get too much in my head, man,” Harmon said of their message. “Injury is part of this game and at this level is part of what we do. So just trying to keep my hopes up and keep me mentally prepared and mentally focused.

“I was still in that head space, but once I got next to my teammates, they cheered me up man. They told me get out of my head and that’s what they really telling me. So that’s why I was laughing like that.”

Harmon acknowledged his frustration with the timing of the injury coming after a strong training camp where he was a starter on the defensive line.

“Definitely frustrated, man,” he said. “Last preseason and really excited to get out there in a regular season and compete at a high level . And it sucks, but it’s part of the game we play. So just got to attack this rehab.”