But former United captain Rooney, who now plays for D.C. United in Major League Soccer (MLS), said the players had to take some responsibility for the situation.
Mourinho remains under intense pressure at United following a poor start to the Premier League season and public fall-outs with key players.
The Sky Sports pundit added that the board had "undermined" Mourinho's position before the start of the season by revealing why they couldn't sign the centre-backs the manager wanted.
"And I have to say that Jose Mourinho will get the biggest support he's ever had in his life tomorrow I think in that ground". The people in the boardroom who are overseeing the football operations are nowhere near good enough; they're not qualified.
"Probably collectively it is a bit of everything coming together, but Jose is an easy target".
United are now eighth in the Premier League and already seven points behind the top-three with only eight games played.More news: Mega Millions posts second largest jackpot ever
The FA have launched an investigation after Jose Mourinho appeared to mouth obscenities into a television camera following Manchester United's comeback win over Newcastle on Saturday.
Mourinho's job is at risk at United following a string of poor results and a public bust-up with World Cup victor Paul Pogba.
Former club captain Rooney says the mood around the club reminds him of Louis van Gaal's hard final few months at Old Trafford, and he wants the players to start performing for their boss.
Speaking to the Telegraph, the 32-year-old English man said some of the Manchester United squad need to be better.
"I said the same thing when Louis van Gaal was there", Rooney said. We didn't promise to each other we would win the match, we promised we would give absolutely everything without fear.
Boss Rafael Benitez said: "I think we did so well for 70 minutes".
The United faithful expressed their frustration in Tuesday's goalless Champions League draw at home to Valencia, cheering the substitution of out-of-form Alexis Sanchez's substitution, chanted "attack, attack, attack" and then booed at the final whistle.