Marshall took a month off from throwing after a mid-July setback and resumed eight days ago.
"I built my arm strength back up and got familiar with my mechanics again," Marshall said. "I got a lot of strength in there last month. The shoulder is really stable. So far, it's been good."
Marshall has increased his long-tossing distance to 105 feet and hopes to soon to be throwing pitches from flat ground. If all goes well, he's hopeful he could be on a mound by the beginning of next week. But will his efforts be enough to get him back to help this season?
"I feel very optimistic," Marshall said. "I'm trying as hard as I can and working as hard as I can every day."
Mark Sheldon is a reporter for MLB.com. Read his blog, Mark My Word, and follow him on Twitter @m_sheldon. This story was not subject to the approval of Major League Baseball or its clubs.