In one instant, the vibe around the Hawks shifted from the possibility of contending for an Eastern Conference title to simply cobbling together enough victories to qualify for the NBA playoffs.

That’s the impact of the torn left pectoral muscle suffered by Hawks All-Star center Al Horford. An MRI on Thursday confirmed the tear, and Horford said he plans to have surgery that would sideline him for three to four months.

Under the most optimistic time frame, Horford would return near the end of the regular season and be ready for playoffs. But if Horford needs the full four months to recover he essentially would be out for the playoffs.

The Hawks have qualified for the postseason for four consecutive years and have won a first-round playoff series the past three years. Now, without Horford, coach Larry Drew couldn’t say if the Hawks can keep that streak alive.

“I sure hope so,” Drew said. “I think we have the talent. The one thing that probably concerns me more than anything is how do we respond to a situation like this. I think given the talent and the depth of our team we certainly can make a playoff push.

“It’s going to be my job to keep these guys mentally strong. This is not a time to start feeling sorry for yourselves.”

Horford’s injury derails what had been a promising start for the Hawks, whose 7-4 record with him included victories against East contenders Miami and Chicago. Horford is versatile and productive on offense and defense and the Hawks’ depth at his center is thin.

Zaza Pachulia replaced Horford in the starting lineup, and Jason Collins is the only other center on the roster. During the preseason, the Hawks opted to round out the roster with guards and rookie forward Ivan Johnson instead of adding another center.

Hawks general manager Rick Sund said the team hadn’t yet decided if it would seek to sign a free-agent center.

“We’ll sit down the next three of four days and talk it over to see if there’s anything we want to do,” Sund said. “Do we want to go with the status quo, or how do we want to do it.”

For now the Hawks will push forward with All-Star guard Joe Johnson and talented power forward Josh Smith as their main cogs.

“We still have enough talent to be an elite team in the East,” Johnson said. “We understand one man can’t do it all. We have to trust each other. We have to muster up a little more to make up for what we’ve lost.”

The injury occurred early during the Hawks’ 96-84 loss at Indiana on Wednesday. He fell to the floor after Indiana center Roy Hibbert blocked his shot, and then Horford’s left arm got tangled with Hibbert as Hibbert pulled away.

Horford was in good spirits while sitting at his Philips Arena lockers before the Hawks played Charlotte on Thursday. He said even as he underwent the MRI, he figured the injury wasn’t serious and might keep him out a couple weeks.

Horford said doctors told him the injury is uncommon for basketball players. It’s the first major injury of Horford’s five-year career.

“It’s one of those things that happens,” he said. “It’s a fluke thing.”

Horford said he thinks the Hawks still can make the playoffs but would need “collective effort” to do it without him. But they will have a tough time winning while missing Horford’s talent, production, versatility and energy.

A two-time All-Star, Horford ranks among the team’s leaders in scoring (third entering Thursday’s game), field-goal percentage (first), rebounds (second), blocks (second) steals (tied for third) and assists (fourth).

“It’s disappointing news,” Hawks co-owner Michael Gearon Jr. said. “Al is a big part of our success. Hopefully he will be back by the playoffs. The tough part is we have been playing hard and playing well. We will have to try to get through it. We’re a veteran team and we have to find a way.”