Rashford repaid the faith with four goals between the two games to ensure United remained in the Europa League and in touch with the Premier League's top four following his brace in Sunday's 3-2 win over Arsenal.
But Van Gaal said he had only seen the 18-year-old in fleeting appearances during training games before he thought to try the youngster as a striker.
"He was training with the second team," Van Gaal said.
"Sometimes he could play the part in 11-against-11 and then I saw him. You can see him with the second team but he was fast and playing on the right and left side.
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"I put him in the striker's position. You never know if a player can cope with the rhythm of your team. He did it very well."
But the Dutchman warned that while he has been impressed with Rashford thus far, the real pressure would come soon.
"The first time it is not so difficult because the player is so focused. Now the pressure shall rise for him because all the people shall look at him in another way," he said.
"After four goals, the fans are expecting the fifth. But he has certain qualities. He can score in his way and that’s because he sniffs the situation in advance of other players."
Rashford's debut only came as the result of injuries to captain Wayne Rooney and then Anthony Martial coupled with Van Gaal's decision not to buy a third senior striker in January.
"I have not bought a striker because of Wayne Rooney and Martial, because they could play in the striker position, and because Rooney is the captain he shall always play. I don't buy a third striker, so he [Rashford] has the possibility," he said.
"He was on the bench at Watford and Leicester. He was having a very good period.
"He might have come in for the Watford match but we played another system and Memphis Depay and Jesse Lingard played very well.
"Because we have a small squad and no more than two players for one position, you can give an opportunity to a youngster."