The Portuguese is under pressure after United's worst start to a league campaign for 29 years.
His side have not won at Old Trafford since the opening day of the season.
"We can do much better. To do that we need points - points we lost in the last two matches," Mourinho said before today's home game against Newcastle.
Tuesday's goalless home draw against Valencia in the Champions League followed defeat at West Ham, a Carabao Cup exit on penalties to Championship side Derby and a draw with Wolves.
Asked at a news conference on Friday if another match without a victory would not be good enough for a club of United's size, Mourinho replied: "Yes, I accept."
The 55-year-old, who was involved in an apparent training-ground dispute with midfielder Paul Pogba last week, said there were "many reasons" for the winless run, but declined to explain what they were.
He did not answer when asked if he could assure fans he was doing everything possible to turn the team's form around.
Paul Scholes has followed former Manchester United team-mate Ryan Giggs in saying United should stick with Mourinho, although the ex-England midfielder told BBC Radio 5 live he is "sick of him moaning".
United are looking to avoid going five games without a victory for only the second time since 1999.
The Derby result - which Mourinho does not classify as a defeat - followed a 1-1 draw with Wolves at Old Trafford, while United lost 3-0 at home to Tottenham earlier in the season.
"Only one defeat at home all season, only one defeat in the past seven matches," he said.
"But no victories at home in the past three matches - three draws. Not the feeling of the defeat, but not the happiness of the victory.
"So, yes, it is important for us to try and win this match on Saturday."
United are 10th in the Premier League, with 10 points from seven games, but take on a Newcastle side seeking their first win of the season.
Scholes, who won 11 Premier League titles and two Champions Leagues with United, has been a strong critic of the team's recent performances - but he believes Mourinho should stay at Old Trafford.
"Yes I agree. I'm sick of him moaning about what he hasn't got, when I think he really should be concentrating on what he has got," he told Robbie Savage's Premier League Breakfast show.
"He's got some real quality in the squad who are very low in confidence.
"Over the years, Jose has been a brilliant coach - his record suggests that. He's been one of the very best.
"I'd like to see him stay, take this - which I think is probably the biggest challenge of his career - and show us what a good coach he is."
Scholes suggested the performance against Valencia was "a little bit different", adding: "They tried, they worked a bit harder but just lacked the real quality to win the game."
He believes Mourinho, who was appointed United manager in May 2016, needs to give players an extended run in the side.
"Unfortunately, because the form's not great, he's chopping and changing. He has no idea what his best team is, he has no idea what his best system is - and this is after over two years," he said.