The Red Devils are still without a Premier League win under the Dutchman after they were held to a goalless draw by Burnley on Saturday.
United were also dumped out of the League Cup at the hands of MK Dons following a 4-0 loss last week.
But Fletcher insists that it will take time for him and his team-mates to adapt to their new manager’s approach.
“We have to master the system and we have to play better and win individual battles,” said Fletcher.
“There are other things that come into it apart from the system. I am confident that we will put things right if we keep believing in ourselves and the manager.
“This is another disappointing result so when we come back from our internationals we are going to have to work on things.
“Everything is new but basic football principles come into it, too. We are trying our best. We just need to keep believing in the manager’s philosophies and ideas.”
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