Ryan Zimmerman. (Photo by Rob Carr/Getty Images)

Ryan Zimmerman did some light throwing early Thursday afternoon, a small step in his long road to returning to the Nationals’ lineup from his Grade 3 hamstring strain. He will repeat his throwing session — increasing distance and trying the different types of throws — depending on how his hamstring feels, Manager Matt Williams said. “It was nice and easy but it allows him to do some baseball-related activity,” Williams said.

Although a small step, it was a good sign that the veteran infielder-outfielder’s leg felt well enough to throw for the first time since hurting his hamstring on July 22.

“You have to use that muscle to throw,” Williams said. “It’s part of the strengthening process and getting him back to some baseball activity and working through that. That’s the early stages of it but it’s a step for sure.”

While there is no set timetable on Zimmerman’s return, Williams has expressed optimism that it will be “sometime in September.” The biggest milestones in his recovery involve hitting and running. According to Williams, the three biggest hurdles for Zimmerman will be taking batting practice, running at full speed and running on the bases. The last of which is “probably the biggest test because he has to react and turn and create different angles,” Williams said.

Grade 3 is the most severe hamstring strain, meaning there is a tear in the muscle. In recent weeks, Zimmerman has been using an anti-gravity treadmill that lightens the strain on the legs while he walks. The Nationals have hoped that the muscle responds well to treatment and strengthening, and that he can return before the regular season ends. The reason for optimism is that Zimmerman’s hamstring wasn’t completely torn or ripped in half, Williams said.

“And it didn’t require them to go in and reattach anything [through surgery],” Williams said. “So a pulled muscle is simply those fibers fraying and they have to grow back together and create some scar tissue and matte back down a little bit. Being in the belly of the muscle wouldn’t necessarily require that.”

The Nationals will be cautious, however, because as the muscle heals it will feel strong but then falter when stressed, and cause an even bigger setback. Zimmerman won’t return until the Nationals are completely sure he is 100 percent, Williams said. As a result, playing either third base, first or left field would make a difference for Zimmerman. “It would affect him equally both places if he was not well,” Williams said.

>>> Outfielder Steven Souza Jr. will begin his minor league rehab assignment on Thursday with Class A Hagerstown and then move it to Class A Potomac. Souza has been dealing with inflammation in his AC joint after he smashed into the right field wall at Turner Field on Aug. 8. He is eligible to come off the 15-day disabled list on Aug. 25.

Souza will progress until he plays a full game. “He’s scheduled for the next week or so and we’ll see how he reacts,” Williams said. “He needs a few games. Sitting around and not being able to swing. He needs to get his timing back.​”

The Nationals can take their time with Souza. Rosters expand on Sept. 1 and minor league rehab assignment last up to 10 days.