Upson made it clear he would not extend his Birmingham deal

Upson, 27, joined the Hammers for an initial �6m on transfer deadline day after asking to leave St Andrews.

Gold told BBC Five Live's Sportsweek: "We feel that maybe he would not have given his all.

"Players underperform when they have things on their mind and Matthew could easily have fallen into that category."

Interview: Birmingham chairman David Gold

Upson had made it clear he wanted to leave the club and Gold says that was the main reason the deal went through.

Of course Steve Bruce was hugely disappointed with this decision and I understand what he feels from a footballing point of view Birmingham chairman David Gold on the sale of Matthew Upson

"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 couldn't keep was the Matthew Upson of the final two weeks who was calling David Sullivan saying 'please let me go, 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 City 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? It is uncomfortable when that player is in your midst."

Gold admitted Blues manager Steve Bruce was unhappy to see Upson leave - but denied the sale had undermined Bruce's authority.

"We have worked very well with Steve over the last five years. It has probably been the best relationship with a manager of our tenure and it is as good as ever," Gold stated.

"Of course Steve was hugely disappointed with this decision and I understand what he feels from a footballing point of view.

606: DEBATE We won't miss Upson at all unless we have injury problems at the back OB 606: Have your say

"What he wouldn't be privy to of course was what was going on in the background with the telephone calls from Matthew and his agent.

"Knowing Steve he will bounce back. We still have some great defenders and if he feels we need another central defender then we will do everything possible to bring one in.

"Promotion is still our prime objective we want to get back to where we feel we belong."

Upson was out of contract at Birmingham in the summer of 2008 and Gold says it would not have been feasible to keep him until the end of his deal.

"It came down to the fact that we felt the player did not want to play for Birmingham City and the time was right to make the change," Gold explained.

"It is down to player-power. I remember a similar situation when Robbie Savage left to join Blackburn when it was more graphic.

"When a player really wants to go, unless you take the stance of saying 'you will rot in the reserves' - which I think is a nonsense approach - you have to make a decision.