Twice the Denmark international put Erik ten Hag's side ahead, but three goals from the visitors ensured United lost our second Champions League fixture of the season.
Following the defeat, which also saw Casemiro dismissed for two yellow cards before Galatasaray's winner, Rasmus urged his team-mates to remain united in order to get back to winning ways.
"We need to remember we can't [concede] goals right after we score," he said. "We [have] had a few games now where they just score, after we scored.
"So we want to go back and analyse that tomorrow and talk about it in the group.