With interim manager Guus Hiddink set to end his temporary spell at Stamford Bridge this summer to return to the Russian national team full-time, speculation is intensifying about the long-term managerial situation in west London.
AC Milan coach Carlo Ancelotti's name has been associated with the soon to be vacant job while Mourinho, who is nearing the end of his first season at Inter Milan, has been linked with a return to the club he left 18 months ago.
However, when asked in a fans' question and answer session about the possibility of Mourinho making a dramatic comeback Buck said: 'My honest answer is that I don't think it is likely to happen.
'Jose was a great coach, he is a great guy but I think his time at Chelsea Football Club has passed.'
Asked about Mourinho's abrupt exit in 2007, Buck added: 'Well first of all we didn't fire him; it was mutual that he left the club.
'There is not much more we can say than in September 2007. The board and Jose were not seeing eye-to-eye on a number of things and it was time we parted company.
'We have great respect, obviously, for what he did for Chelsea Football Club, we are on great terms with him. I saw him at the Manchester United match, he is a great guy and he did great things for the club.
'But in September 2007 Jose and the board and Roman decided it was time for Jose to leave.'
Buck also emphasised Hiddink's tenure is strictly short-term.
'Guus Hiddink came here on a temporary basis two months ago and it is thanks to the Russian FA that he was able to come here; it is purely a temporary role,' said the American.
'There has been a lot of speculation about him staying, particularly as he has been doing a great job. That will not be the case.
'It is now up to the board to find a new manager for the long term and that is a process that we have started and is ongoing. Anything you might read is just speculation at this stage. When we have an agreement for our new manager we will announce it.'