Sullivan has been speaking to the Sunday Times about the decision to part company with Big Sam back in May.
But the wheels fell off spectacularly as Sam Allardyce's side joined Southampton and Aston Villa as the only clubs in the Premier League era to be in the Champions League places by Christmas and not finish there.
Second half of the season syndrome cost Allardyce dear
Allardyce had won praise for turning things around in east London after an uninspiring 13th place finish the season before.
Fans were left calling for change as the Hammers picked up just three wins in 2015 dropping like a stone down to 12th place.
And now Sullivan has admitted he felt he had to make a change or risk being relegated under the man widely regarded by pundits as 'as near as you can get to a guarantee of survival'.
“We collapsed at the end of last season," Sullivan told the Sunday Times.
"Three wins in 21 was worse than relegation form. We knew we needed changes. There were a lot of warning signs. Fulham had similar the season before, didn’t do a lot, stayed with their manager and got relegated.
“Sam wanted a break. In January or this time next year, he’ll go somewhere and do a very good job."
Sullivan could make pundits eat their words
From the minute Sullivan and Gold parted ways with Sam Allardyce in the summer a number of football pundits have been very outspoken on why the little old Hammers should be careful what they wish for.
Big Sam was a virtual guarantee to stay in the league, and that according to pundits such as Alan Shearer, Graeme Souness, Jamie Carragher, Richard Keys, Alan Brazil, Robbie Savage and many others is the height of the club's ambition.
Bilic was a risk they said. West Ham will struggle without Allardyce they said.
Many West Ham fans have pointed out after the brilliant win over the Gunners that had Allardyce been the manager they would have played five flat across midfield and one striker with a very clear intent to play for a clean sheet and a draw.
To see Bilic play Sakho, Zarate and Payet together, away from home let alone at the Emirates may be all the proof Sullivan needs to feel part the way vindicated already.