West Ham United have high hopes of promotion from the Championship but the severe cost of failing in that aim is as much of a motivation as the desire to make an immediate return to the Premier League. The co-chairman David Sullivan understands not just the allure but also the harm done if the club fails to get there. He knows that the goalkeeper Robert Green, an England international, would then be restless – just as Scott Parker was before his move to Tottenham at the beginning of the season.

"Green's contract is up this summer," said Sullivan. "We have made him a fantastic offer, subject to us [getting promoted and] staying in the Premier League. But if we don't stay in the Premier League, we won't be able to afford him and he won't want to stay. So we have to go up."

Sullivan, who took control of the club with David Gold in January 2010, has spoken frankly about his time at the club in an interview for westhamtillidie conducted by the West Ham fan and political blogger Iain Dale. The views of Sullivan are frank. He is scathing about Avram Grant but also scolds himself. "We picked a bad manager," said Sullivan. "Simple as that. We are as much to blame as the manager."

Grant could not preserve West Ham in the top tier but Sullivan recognises other factors, citing Demba Ba and Parker. "Those two alone should have kept us in the top flight," he said, lamenting Parker's plea for a move at the start of this season.

"This might not please many people but look at Scott Parker's performance in the first five games of this season. To me it didn't look like he was doing the tackling you'd normally expect him to do," said Sullivan. "He was running about, he was doing OK but he wasn't doing the tackling we know he can do because he knew that the one thing that would stop a move for him would be an injury.

"To me it looked like he didn't want to be there. He told us he didn't want to be there. There's no player who has given more for the club in recent years, so to make him stay against his wishes … He was protecting himself either consciously or subconsciously. He's gone to Spurs and he is back to his old self."

In Ba's case Sullivan admits allowing the striker to leave for free in the summer was one of his biggest mistakes. West Ham paid little more than an initial £400,000 to the German club Hoffenheim for Ba because of a medical issue, with further sums related to appearances, but his value has since soared at Newcastle where he has scored 16 Premier League goals in 21 appearances. "It's probably one of the worst mistakes I have ever made in my life," he said. "It just didn't enter our heads. It didn't enter anybody's head that he'd score enough goals that we'd want to give him £40k a week and his old club £15-20k an appearance and we'd still be relegated."

Sullivan claims Newcastle are in danger of losing Ba for as little as £3.5m at the end of the season. "I'm told he's got a £7m get-out at Newcastle and he gets half the money, so if they sell him for £7m, they'll only net about three," he said. "Getting £3m is not bad but, for a player of his quality, it's not fantastic."

Sullivan states that £45,000 a week was offered in the close season but Ba chose to go to Newcastle United. The player had not experienced a soothing environment at Upton Park. "I think there was a foreign group and an English group," Sullivan says of the squad.

"I think the English group were plotting against the manager." He adds that Sam Allardyce, the current manager, "wouldn't have stood for it" had he been in charge.

Sullivan is concerned with more than team-building. There is also the issue of Upton Park and the possibility of taking up residence in the Olympic Stadium instead. He is cautious on the topic. "We have a stadium that could have a 45,000 capacity," he said of the present home. "We have a hotel. We have 3,000 corporate hospitality guests, a stadium we own. We say how, when and who, so to give that up we have to be 100% sure the deal is right for the club."