Louis Van Gaal said He will be using Di Maria up front as a striker as His speed is needed to create more space in the pitch for His players.
Di Maria, 26, scored three goals in his first five games for United after his £59.7 million move from Real Madrid.
He has managed one in the last 10 and Van Gaal's decision to move the Argentine up front has been criticised. (external)
"I tried it because I didn't have the preparation at the beginning of the season," said Van Gaal.
"I tried him as a striker as we needed pace in that position. We must stretch the pitch."
The Dutchman added: "Then you make the pitch bigger, your midfielders can play in a bigger space and play a better positional game.
United's next four games
23 January Cambridge United (a)
31 January Leicester (h)
8 February West Ham (a)
11 February Burnley (h)
Fans chanted "4-4-2" at half-time during United's Premier League game at Queens Park Rangers on 17 January.
The Dutchman had initially deployed a 3-5-2 formation. But after he switched to 4-4-2 at Loftus Road his side went on to win 2-0.
As United prepare for the FA Cup fourth round trip to Cambridge on Friday, Van Gaal defended his tactics after Gary Neville echoed concerns (external) of United supporters over the formation.
"I know all the stats," he said. "I know that we have won more with 4-4-2 in a diamond."
And Van Gaal does accept his preferred system is taking some time for his players to get used to, and added that he would like his players to feel 'more comfortable' in their roles.