Woefully out-of-form Chelsea have already lost 11 matches in all competitions this season but need only a draw against Porto to qualify for the knockout stages.
Former Manchester United great Ferguson said Russian owner Roman Abramovich must ignore the criticism the club are receiving and keep faith in Mourinho.
“He has sacked so many coaches in the last 10 years that I am sure he has learned by it,” Ferguson was quoted as saying by the Daily Telegraph.
“He has to trust and have confidence Jose can turn it around. There is no point in sacking one of the best coaches of all time, he's won the European Cup twice, he's won the league in each country he's managed in, he's won the big trophies.
“It would be foolish to take that step to sack him. That would be bad management,” added Ferguson.
Defeat on Wednesday and a consequent drop into the Europa League for Chelsea would certainly test Abramovich's patience.
After a 1-0 home defeat by promoted Bournemouth on Saturday, Mourinho said for the first time that finishing in the top four of the Premier League to qualify for next season's Champions League was looking unlikely.
Chelsea, who are 14th in the Premier League with only four wins from 15 matches, are likely to have captain John Terry back after injury.
With Dynamo Kiev expected to win at home to Maccabi Tel Aviv, who have lost all five group matches so far, a draw for Porto would eliminate the Portuguese side unless the Ukrainians fail to win.
It was at Porto that Mourinho made his reputation, winning the 2003 Uefa Cup and the 2004 Champions League.
Although the Chelsea fans have remained supportive, the stadium is likely to be tense, at least until their team make a breakthrough.
“It's difficult for us to score,” said Mourinho after the Bournemouth game. “Scoring goals is a lot about individuals.”
Spain forward Diego Costa netted their only goal in the previous three Premier League games but he has been lacklustre and did not start either of the last two matches, with Eden Hazard playing as the main striker.
Porto, who beat Chelsea 2-1 on Matchday Two, had hoped to tie up qualification in the previous round of games but slumped to a 2-0 home defeat by Kiev.
They are second in the Portuguese League, two points behind Sporting Lisbon, after coming from behind at the weekend to beat Pacos de Ferreira 2-1 with goals by Jesus Corona and Miguel Layun who is on loan from Premier League Watford. – Reuters
Original source: Fergie backs Mourinho