The Red Devils have been underwhelming in the opening months of the Premier League season following four victories in their opening eight games.
Manchester United welcomed winless Newcastle United to Old Trafford last weekend, having lost to Brighton, Tottenham Hotspur and West Ham United.
Mourinho was contending with intense speculation about his future at the 20-time English champions ahead of Saturday evening’s clash against the Magpies.
Newcastle surged into a 2-0 lead inside 10 minutes to create a toxic atmosphere at Old Trafford as Mourinho was plunged into a deeper crisis.
However, Juan Mata, Anthony Martial and Alexis Sanchez all got on the score-sheet in the second half to rescue a 3-2 win.
Manchester United have produced some turgid performances under Mourinho this season, with key players such as Paul Pogba and Sanchez struggling for form.
Former Red Devils skipper Rooney defended his ex-manager Mourinho and criticised the Manchester United squad for a below-par start to the season.
“The manager can do so much but then it is down to the players on the pitch to produce and probably collectively,” Rooney told the Telegraph.
“It [the form] is a bit of everything coming together but Jose is an easy target. Some of the players have to be better.”
Rooney added: “I said the same thing when Louis van Gaal was there.
“He took a lot of stick but behind closed doors I said to the players, ‘we have to be doing better’.
“Personally I think he set us up brilliantly but we didn’t produce on the pitch so I am sure that is getting said behind the scenes.”
Manchester United are in eighth place in the Premier League table and seven points behind the leading pack of Manchester City, Chelsea FC and Liverpool FC.
For all their struggles, the Red Devils could move to within three points of Mourinho’s former club Chelsea FC if the 20-time English champions win their next fixture.
Indeed, Manchester United will make the trip to Stamford Bridge in their next Premier League fixture on Saturday 20 October.
While Mourinho’s side have been struggling in the Premier League, the Red Devils have collected four points from their opening two Champions League fixtures in the group stage.
Manchester United were 3-0 winners at Young Boys before a goalless draw with Valencia ahead of their back-to-back fixtures against Juventus.
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