"I think, anytime Calvin says something, it's not for show, it's something serious," Stafford told The Mitch Albom Show on WJR-AM. "I think you guys have covered him long enough to know that he's a pretty measured person, and everything that he says to the public is a thought-out thing. So in that regard, I'm sure it's thought-out."
Johnson can make $16 million for playing in 2016, when he'll turn 31 on Sept. 29.
"Like many players at this stage of their career, I am currently evaluating options for my future," Johnson said in a statement earlier this month. "I would expect to have a decision regarding this matter in the not-too-distant future."
Johnson said following the Lions' Week 17 win at Chicago that he did not know what the future holds for him. He is expected to meet with new general manager Bob Quinn before March, who will need to know Johnson's plans entering the 2016 league year that month.
"He's a heck of a player," Stafford said. "We'll see where his head is at and what he's thinking."