The Pompey defender, 33, had a public bust-up with chief executive Peter Storrie earlier this week and did not feature in the 3-1 win against Derby on Saturday.
But the club last night insisted Campbell's absence was down to a back injury.
A Portsmouth spokesman said: "It's quite simple. Sol had a back problem and he's been having treatment on it all week. We hoped he would be fit but he couldn't make it. I spoke to Peter Storrie about it and he laughed and said it was rubbish."