Just four days after slipping on a puddle and getting stitches, the buff That Awkward Moment star, 26, was spotted working out in Venice, Calif., on Thursday evening.
"He looked great," an onlooker tells PEOPLE. "But it was all business. He was concentrating on hitting the weights hard."
Wearing long sleeves and nylon pants, the former High School Musical heartthrob sported what looked like a fresh haircut slicked over to the side as he used a rowing machine to perfect his deltoids.
"He seemed really pensive and was looking around to see who was looking at him," the onlooker says of the actor, who completed a stint in rehab earlier this year.
While Efron showed no signs of his recent injury, the onlooker noted that he was not seen moving his mouth, which was wired shut after the accident.
The buff star has been recovering with a little help from his friends, including Hairspray director Adam Shankman, who has been bringing Efron soup.
"He has to eat through a straw on a syringe. It's horrible," Shankman told PEOPLE at the GO GO Gala in Los Angeles on Thursday. "He's in great spirits. He's just like, 'Wow. It hurts.' His jaw is wired shut."
Shankman has also given Efron some good-natured tough love. "He's like my little brother, has been since Hairspray," Shankman said. "'So I talk to him like a little brother, which means I give him a lot of s––."
"He has always been a klutz," the director added. "And this is like a perfect Zac move."
With reporting by REAGAN ALEXANDER