This story was excerpted from Steve Gilbert’s D-backs Beat newsletter. To read the full newsletter, click here. And subscribe to get it regularly in your inbox.

PHOENIX -- On his daily visits to Salt River Fields to go through the rehab process, Zac Gallen has plenty of company. Fellow rotation mates Merrill Kelly and Eduardo Rodriguez are there as well, each working their way back from injury.

During Spring Training, the trio was supposed to form the front three of the rotation. But then Rodriguez strained his left lat muscle towards the end of camp and re-injured it weeks later when he tried to come back too soon, and Kelly suffered a strained teres major muscle in mid-April and had to go on the IL.

Gallen joined the group after straining his right hamstring in his May 30 start against the Mets in New York.

Though he was the last one hurt, Gallen’s injury was the least severe, so he is expected back sooner than the other two. Friday, Gallen threw a 37-pitch bullpen session and he is set to throw a simulated game Tuesday.

“It felt pretty good to me,” Gallen said. “Everything felt like it was coming out pretty good. I felt sharper my first one the other day, but still felt pretty good with how I felt today. So I think I’m progressing pretty solidly so far.”

Whether Gallen needs another sim game after Tuesday before being activated has yet to be determined, but while Kelly and Rodriguez’s injuries kept them from throwing, Gallen was able to keep his arm in shape by playing catch just a couple days after going on the IL.

That means it will take him far less time to build back up for a return.

“I think two days after the injury I threw some weighted balls against the wall, so my arm really never stopped moving,” Gallen said. “And then I got to play catch a couple of days after that. So in terms of buildup, I really never stopped throwing. The hamstring was sore for a couple of days after I got hurt but after that I didn’t really feel anything and nothing has popped up with it since, so that’s been good.”

It's helped to have company in what can be a lonely rehab process.

“Being in rehab can be monotonous, but I think guys’ spirits seem to be up and they’re progressing well,” Gallen said. “I mean obviously everyone’s goal is to get out of there as soon as possible and as healthy as possible, but it’s been good so far.”