Tyler Blackett came on in his place and Van Gaal also introduced James Wilson for Falcao, who received a standing ovation for his efforts.
Osman broke through the United defence in the final minute, but once again De Gea rescued the hosts with a fine diving save.
The Spaniard denied Oviedo with another acrobatic save and the match ended on a bad note for Everton when John Stones was carried off on a stretcher with an injury.
