Van Gaal brought Fosu-Mensah on for the injured Marcos Rojo during the second half at Old Trafford.
The 18-year-old had previously impressed in the U21s under Warren Joyce at centre-back and in midfield but he performed well in an unfamiliar role on the left against the Gunners. Van Gaal revealed that he sees Fosu-Mensah as a left-back in the long-term.
“I was thinking for a long time that he could play better as a full-back. I said that to Warren and Warren has tried him out in the second team, and I think he did it very well. We have problems at the full-back positions and that is why,” van Gaal said.
“Then he came in against Arsenal and did fantastically. He didn’t make any mistake and that is fantastic.”
It was fantastic to see United dominate the game with so many young players on the pitch and the result gave supporters optimism about the future. Van Gaal has been criticised this term, and rightly so, but he has done an excellent job in bringing youngsters through.