McPhee spent the first six weeks of the regular season on the physically unable to perform list after offseason knee surgery.
He returned to practice last Friday but only participated in three walk-through workouts before the club added him to the 53-man roster. He missed the entire offseason program and training camp because of a knee procedure that teammate Willie Young said "used to be career-threatening" for some players.
It's unclear what impact McPhee can have against the Packers or how much he realistically can be expected to play with such short preparation time.
But McPhee did not sound concerned about his health when he spoke on Tuesday.
"If you watch me walk, you can tell, you know what I'm saying?" McPhee said. "But I'm telling you: I'm blessed, I'm healthy, I breathe real good, you know what I'm saying. But y'all will see, bro. Time will tell. Trust me."
Despite dealing with a lingering knee injury much of last year, McPhee still finished tied for second on the Bears' defense in tackles for loss (10) and third in sacks (six) in 14 games. He played for Baltimore from 2011 to 2014, winning a ring with the Ravens in 2012.
"[I'm] just a nasty, hard-nosed, going-crazy-on-the-field type guy," McPhee said. "Just physical, man. I'm just going to bring a physical impact, man."
McPhee is the Bears' highest-paid defensive player with a $7.675 million base salary in 2016.