Media reports claim Moores will be dismissed when Andrew Strauss becomes England's director of cricket.
Flintoff, who played under Moores during the 52-year-old's first stint as England coach, wrote on Twitter: "His agenda is England winning, he needs the players to hold up their end too."
Moores' side drew their recent Test series in the West Indies 1-1.
Former Lancashire all-rounder Flintoff, 37, said it was not Moores' fault England "underperformed", adding he hoped the reports about his sacking were not accurate.
Moores, reappointed England coach in April 2014, was in charge for a disastrous World Cup campaign, which saw them eliminated by Bangladesh.
He also oversaw the draw with a West Indies side labelled "mediocre" by incoming England and Wales Cricket Board chairman Colin Graves.
Stand-in captain James Taylor said Moores was "positive and bubbly" during Friday's one-day international against Ireland, despite the speculation about his future.
Some reports on Saturday said Moores' dismissal would happen before the expected appointment of former England captain Strauss, who retired in 2012.
Former England opener Geoffrey Boycott wrote on Twitter that ex-Lancashire coach Moores was "a nice man" but "out of his depth at international cricket".
Boycott also questioned the selections and tactics of Moores and captain Alastair Cook during the recent series in the Caribbean, which concluded with a five-wicket defeat inside three days at Bridgetown.
Moores first led the national team between 2007 and 2009 but was sacked after being involved in a dispute with batsman Kevin Pietersen.
England coach Peter Moores is a "great man" who "deserves better", says former national captain Andrew Flintoff.