"Buyer beware," Lynch said on the "Waddle & Silvy" show on ESPN 1000.
Lynch, who retired last year and will be a FOX television analyst this season, was Marshall's teammate in Denver during the 2006 and '07 seasons. Marshall had made some noise about wanting to be traded, but he recently said he will report to training camp on time.
"I don't like saying [buyer beware], but you've got to call a spade a spade, and I think it's a dangerous proposition," Lynch said. "I don't think you'll see the Broncos sign him to a long-term deal, because right now the behavior he's demonstrated off the field, I don't think you'd feel comfortable doing anything long term.
"As I said, on the field, he's very comparable to a guy like Terrell Owens. He's that good of a player. But being a professional in my mind ... takes doing it on the field and off the field. You can't have all choir boys. I believe in first and second chances, but when you're talking about 13 or 14 ... I think at a certain point you've got to say this guy's got some issues that he's got to work out before you can trust him."
Marshall has one more season on his Denver contract. He's been arrested four times -- including once for domestic abuse -- since March 2006, and he has been penalized with one suspension.
Lynch was more complimentary of his former teammate Jay Cutler, whom the Bears acquired in an offseason trade with the Broncos.
"I like Jay, he's a friend of mine," Lynch said. "He had some growing up to do, probably still has some growing up to do. But in saying that, I think the Broncos erred in letting him go.
"This is a guy, in my mind, that's a once-in-every-15-year-type talent. He's got that kind of skill. I think he'll grow into the other things. I sometimes think during this whole ordeal, they tried to paint him as a bad guy, but he's not a bad guy at all. He's got some growing up to do, but hopefully he learned something from this whole ordeal and offseason."
Lynch said part of that maturation process for the 26-year-old Cutler is opening up.
"I think he's very reserved and to himself, and that's fine," Lynch said. "I think sometimes it's just the way -- I hate to put it on this -- but sometimes the new generation. They just don't understand things that you think everyone should understand. I think he wants to learn. I think he's reached out to people trying to learn, and I think he will because as I said, once you get to know him on a one-on-one basis, he's by no means a bad person or a bad teammate. He's just, as I mentioned, got some things to grow up in."
Lynch, who was born in Hinsdale, Ill., predicted the Bears would win the NFC North.
"You guys are in Chicago right? I was born in Hinsdale, so I better say Chicago," Lynch said. "Lovie [Smith is] a good friend. I think what bodes well for the Bears is Lovie getting back involved in that defense.
"I think at times you feel like, 'I just need to be a head coach,' but when you don't see the type of play that you're accustomed to seeing as a coach, I think he's as good as it gets as a defensive coach in this league, and he's going to get back involved. He's got a guy, Rod Marinelli, don't underestimate him. In our years in Tampa, we put more pressure on quarterbacks than anybody, and he was a big reason. You've got to have the players, but he gets the best out of each and every player. So the Bears, there's my pick."