"On my best days, I feel like I'm 80 percent of who I used to be," Marshall said. "I feel like I just can't quite get back to my old self."
The Reds previously activated Marshall on April 19, but he wasn't as effective as he had been in 2012, his first season for the club.
In 15 games, totaling 14 1/3 innings, Marshall had a 7.71 ERA with 23 hits, 12 earned runs, 12 walks and 14 strikeouts. His fastball velocity has been down in the low-to-mid 80s, from its usual upper 80s and 90 mph. He retired only five of 15 first batters faced and allowed five of his 11 inherited runners scored.
"You see that it's not just the results, but the stuff hasn't been great," Reds manager Bryan Price said. "It's just not getting where it needs to be for him to get back to being the guy he's been.
"He's really battled this issue for a while now, so I think we're going to try to go back to square one and reassess and see if there's been any changes in the shoulder that would bring on this current soreness, if it's related to an old injury or it's something new, and reassess what options there are."
Marshall pitched back-to-back games on Monday and Tuesday vs. the Dodgers for the first time this season.
"I threw a couple of fastballs to [Dee] Gordon there and the second one, I pulled up again. I felt that pinching soreness in the back of my shoulder. It's feeling better, but I'm not myself," Marshall said.
"We thought it was a good idea to try other things with my shoulder. I've tried a lot of things. I've tried acupuncture, creams and all sorts of stuff, and all the strengthening exercises you could think of. I feel like it's a good time with Mat coming off [the DL] and Tony being the long guy. I've lost some sleep thinking about some things, thinking about if I'm helping or hurting the bullpen."
Marshall, who is signed through 2015, is earning $5.5 million this season and is set to make $6.5 million next season. He is trying to avoid undergoing surgery as much as possible.
"That's the last thing that I want," Marshall said. "I've heard stories where you invade the shoulder, it just doesn't want to bounce back. It's a 50-50 chance that you're back to who you are again."