Campbell made his League Two debut for Notts County in Saturday's 2-1 defeat at Morecambe but left the club by mutual consent five days later.
The 35-year-old former Portsmouth, Arsenal and Tottenham star had arrived as a free agent last month on a deal reported to be worth £40,000 a week.
It was a real statement of intent from the ambitious new owners at Meadow Lane, with Eriksson playing an instrumental role in persuading Campbell to drop down to the Football League's bottom tier.
But the move quickly turned sour, with reports suggesting Campbell had become disillusioned with the club's rate of progression during his short stay.
Eriksson claims not to know why the player walked away from the club, but insists that reason would be grossly unfair should it be true.
We are all sorry that he has gone but the project goes on without Sol Campbell, nothing has changed
The Swede continued: "I really don't know the real reason. He didn't like the training pitch and the dressing room and things like that but he knew that before because we showed him around before he signed.
"We know we are not perfect at this moment and I don't think that would be fair at all. It is a long-term project.
"Since the new owners came in, the club have signed seven or eight new players - and you can't build a new training ground in four or five weeks.
"It takes time to make the stadium better, it can't be done over a weekend so I think that's unfair.
"We are all sorry that he has gone but the project goes on without Sol Campbell, nothing has changed."
County manager Ian McParland also expressed his dismay with the departure of Campbell.
"I'm disappointed because I got on well with Sol. He's a gentleman, he's a nice man and the lads got on really well with him," he told BBC East Midlands Today.
'Resilient' Notts can cope without Campbell
"He was in that dressing room and mucking in with all the boys.
"I enjoyed working with him, he was never a problem. I'm disappointed he's not stayed around but that's life. People come and go from football clubs all the time.
"The fans will be disappointed - there's never been a star like that here for a long time... an England captain coming here was fantastic for them."
Campbell's own future remains uncertain.
Capped 73 times by his country, he is now highly unlikely to be allowed to play for any other club until the January transfer window.
But former manager Arsene Wenger thinks he could still play at the top level again.
"It depends how much he wants it," said the Arsenal boss. "He still has enough in the locker to play in the Premier League. If he wants to play in the Premier League, he can still play in the Premier League."
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