Coughlin said Smith will be on a "week-to-week" status, but indicated he won't plot a definitive timetable for the receiver's return until he begins rehabilitation. Coughlin said no surgery will be necessary.
"It's a partial pec, and he just has to rehab," Coughlin said. "But whatever the basis of it is, he has to go through a process of healing, so I'm not sure exactly how long."
The injury occurred during drills Thursday when Smith was reaching for a pass.
Moving Manningham into Smith's spot could lead to more double-coverage for Nicks, an athletic and shifty weapon in the open field. But adjusting to defenses won't be the only pressure the 2009 first-round draft choice will have felt this week.
Nicks' alma mater, the University of North Carolina, is under NCAA investigation, and the school listed the receiver's name among five former Tar Heels who might have given some of the 13 currrent players under scrutiny $3,300 worth of "impermissible gifts of cash and jewelry and impermissible assistance" with transportation after the 2008 season.
"I'm not angry at all," Nicks said, when asked his thoughts on North Carolina bringing his name into the investigation. "That's college. I'm in the NFL now, and I'm focusing on football."
The investigation also involves improper conduct from a tutor and improper relationships with agents. But the school has stated that Nicks isn't suspected of having any "inappropriate relationships with agents, prospective agents, or runners."
"I've got a game this week," Nicks said as he shook his head to all further questions about the investigation, and proceeded to talk about the Cowboys, and Manningham's upside for this weekend.
"We know he's going to step up. We pretty much all know each other's routes, so it shouldn't be a problem."
Manningham, third on the team with 25 catches for 385 yards and three touchdowns, said he looked forward to an increased role.
"I've just got to play football," he said, "stick to what I do and don't try to make the game harder than it is. The good thing is, we know Dallas and we know what they're going to run."
The Giants might be using that starting group for a while. At least two weeks, according to Coughlin.
"They say a limited amount of time, but not necessarily right away or this week or next week," Coughlin said. "I won't know until (Smith) starts rehabbing. He hasn't had much swelling or discomfort, so a lot of it in the beginning will probably be preventative to make sure there's no further injury, and then to strengthen and go forward."
Notes: OT David Diehl (hip and hamstring), FB Madison Hedgecock (hamstring), C Shaun O'Hara (foot) and KR Darius Reynaud (hamstring) were ruled out for Sunday, along with Smith. OT Will Beatty (foot) is questionable. DE Osi Umenyiora (knee), RB Brandon Jacobs (illness) and LB Gerris Wilkinson (hand) are probable. ... O'Hara told the New York Daily News on Friday that the Lisfranc sprain in his right foot isn't healing as quickly as he'd hoped, and it could take as many as eight weeks to heal. He already has missed one game because of the ailment. ... Coughlin said he'll wait until game time to decide whether veteran Kevin Boothe or rookie Mitch Petrus will start at the left guard spot left vacant in the offensive line shuffle to compensate for O'Hara's and Diehl's injuries. All signs point to Boothe, however.
The Associated Press contributed to this report.