Campbell cannot see an outright favourite for the Premier League title this term.
He told talkSPORT: "No one yet, no one.
"Liverpool, obviously they've been champions, they're always going be in with a shout, and Arsenal are the chasers. They're in a good position at the moment, but they just have to...
"The proof's in the pudding, they have to keep going, keep going when it's January, February time, when it's wintry and raining, away, picking up points.
"So at the moment, it's so early but they've got great signs. Arsenal are looking good, but they've got to believe they can do it, that's the thing.
"If you don't believe you can do it, something's going to come up, and then you're not going to get the points, you're not going to have that total belief.
"Everybody's got to believe they can do it and when they get that mental kind of herd mentality when everyone believes it... not stupidly, like far-fetched, but, 'We've got a great chance and we've got to believe.'
"If you add too much doubt in it, it takes a little bit, one per cent, and that one per cent added up through the whole team, the whole squad, can actually take away that momentum. So they've got to truly believe they can do it, they've got a chance to do it."