Birmingham chairman David Gold felt Matthew Upson had to be sold to West Ham because 'he would not have given his all' had he stayed at St Andrews.
The defender left the promotion chasers to join the Hammers' fight for top-flight survival just before the transfer window closed in a £6million deal.
The 27-year-old - reported to have a get-out clause in his contract should the side go down - had made it clear he wanted to leave Birmingham, having signed from Arsenal in 2003.
Blues boss Steve Bruce was not impressed with the decision to allow arguably his best player to leave.
Gold, however, maintains the board felt they had little choice but to push the deal through.
'We feel that maybe he would not have given his all,' Gold told BBC Five Live's Sportsweek Programme.
'People would have known he wanted to leave, and players would have known.
'We just felt in the final analysis, it was best that Matthew moved on.'
Gold added: 'Players underperform when they have things on their mind and Matthew could easily have fallen into that category.
'We desperately wanted to keep Matthew Upson, the consummate professional who had played for Birmingham for four years and did a magnificent job.
'But that is my point. That was the Matthew Upson we wanted to keep - what we could not keep was the Matthew Upson of the final two weeks who was calling David Sullivan saying 'please let me go'.
'We also had his agent bombarding us with calls, and in the final analysis we felt it was in the best interests of Birmingham to let him go.
'Do you want a player at your club who really does not want to play for Birmingham and who desperately wants to leave?'
Gold insists the board share Bruce's 'disappointment' at Upson's decision.
He said: 'It is player power - I remember a similar situation with Robbie Savage.
'When a player really, really wants to go, unless you take the stance of `okay you will rot in the reserves', which is a nonsense approach, you have to make a decision.'
Gold added: 'The board have had a fantastic relationship with Steve Bruce over five years.
'We have supported him through thick and thin. My relationship with Steve is as good as ever.
'Clearly he is disappointed with this decision - and I understand that from a footballing point of view.
'But what he would have not been privy too was what was going on in the background, with calls from Matthew Upson and the issues with his agent.'
Birmingham drew 1-1 at Colchester yesterday, and are fourth in the Coca-Cola Championship, but with games in hand on the side above them.
Even though the transfer window is shut, the club can still take players on short-term loans.
Gold said 'If Steve feels his requirement is he needs another central defender, then we will do everything possible to bring one in.
'Promotion is our prime ambition.'
Upson, meanwhile, lasted just 28 minutes of his debut for the Hammers at Aston Villa before he was forced off with a calf injury.