Manuel returned to the practice field on Sunday, but only took part in individual drills.
"He went out there and did a nice job," Marrone said. "We're going through a process. (There's) nothing right now that seems to slow him down."
Marrone said the Bills will make a decision Wednesday on who starts against the New England Patriots in Sunday's opener.
"A couple things that we're testing as the week goes on to see where he's at," Marrone said. "Mentally, we feel fine. It's just a matter of physically."
Manuel and undrafted rookie Jeff Tuel split first-team reps during team drills on Monday.
"We're not worried, because we're kind of doubling the reps. We're not getting short-changed anywhere as we go," Marrone said. "We'll probably have more reps than the first team will during the season."
Bills players are off Tuesday before returning to practice Wednesday.