Mourinho is expected to leave Real Madrid at the end of the season and has long been linked with a return to Stamford Bridge, where interim boss Rafael Benitez is under contract until May.
Asked if anyone had been ruled out and about Mourinho in particular, Buck told Al Jazeera at the Business of Sport summit in Singapore: "I am completely open minded about it".
The Portuguese would be the fans' choice, having steered Chelsea to two Barclays Premier League titles, one FA Cup and two League Cups during three years in west London before departing in acrimonious circumstances in September 2007.
Chelsea are still to draw up a list of the possible 10th manager of Roman Abramovich's 10-year ownership.
"We really haven't started that yet," Buck added. "We're all thinking about it and have some ideas and certainly Mr Abramovich is thinking about it. At the moment we're concentrating on the end of the season."
Benitez was appointed in November after Champions League-winning boss Roberto Di Matteo's departure when the Blues became the first holders of the trophy to exit at the group stage.
Buck has been chairman since Abramovich's takeover in 2003, with the Russian oligarch's reign coinciding with great success, despite managerial turmoil.
Buck said: "I know we have fired what most people would say are a lot of managers - terminated the relationship is a better way to describe it - but we've always thought long and hard when we've done it.
"It's always difficult, it's always sad when a relationship is terminated. We don't look back, we always look forward and see where we're going and figure out how we get there. We certainly believe in stability in managers, but it has to be with the right manager."
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