The Giants were 5-0 in the preseason while adapting to the West Coast offense under offensive coordinator Ben McAdoo.
"Don't get me wrong, we're ready for Week 1," Manning said Friday. "But I think as a season goes on, you're always looking to improve, whether you've been in a system for 10 years or whether you have young guys and need guys to step up or you have new players, there's always room for improvement.
"I don't want to alarm people and say, 'We've got to get better. We've got to keep getting better.' That's just part of it. We're going to keep getting better, but we can still go out there and be successful and do good things and win games."
The most immediate issue involves just who will help the Giants do those things. The final roster cuts from 75 to 53 are due Saturday at 4 p.m., and undrafted rookie receiver Corey Washington will spend a long day waiting by the phone. He, along with fellow young receiver Preston Parker and Super Bowl hero Mario Manningham will be hoping to avoid an unwanted call.
Manningham injured a calf Thursday night against New England, ending an ineffective, two-catch preseason, and Parker was productive but unspectacular.
"It's going to be tough," said Washington, who had 10 catches for 155 yards and four touchdowns in the preseason. "It's another wait. I've done all I can do. It's a matter of waiting right now. ... I found a couple of roles on special teams. I didn't have any tackles, but I set guys up. I wouldn't be disappointed being on the practice squad. Practice squad, active roster, I'm here to help the guys get wins."
Free-agent left guard Geoff Schwartz will have to await his turn to help after dislocating a toe. Because surgery was never an option, according to Schwartz, he could be placed on short-term (eight weeks) injured reserve. While Tom Coughlin put Schwartz's rehab at three to four weeks, the coach admitted it could take more time.
"I would add a little more to that, just to be safe," Coughlin said.
There is no real timetable because the severity of the injury tends to increase with a person's size. Schwartz weighs in at 340.
"This is a unique type of thing," Schwartz said. "Especially with toes, and with big guys, you just have to see how the rehab goes. There are a lot of different ways to approach it. Being a bigger guy, putting pressure on that toe is not going to help."
Cornerback Prince Amukamara could be in the lineup against Lions Pro Bowl receiver Calvin Johnson and new additions Golden Tate and Kevin Ogletree, after being cleared to practice Monday. Amukamara has sat out since the third game, when he injured his groin against the Colts.
"The more I practice, the more confident I'll get," Amukamara said. "I got the opportunity to face (Johnson) last year. I'm just going to study all their guys."
For Manning, Detroit will represent a huge first test.
"I think we can score points," he said. "I think we've done some things. We haven't shown everything in the book. We haven't thrown the ball down the field a whole lot, so I think there's definitely room to score some points and move the ball and be successful. It's just a matter of doing it."