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Rangnick spent the second half of this season as United interim manager and was set to move into a consultancy role this summer.
He took the Austria role in April and was initially going to balance the two jobs. Last week saw United announce Rangnick had committed his future to Austria.
The 63-year-old couldn't have asked for a better opening match as Austria ran riot in the Nations League.
Marko Arnautovic opened the scoring in the first half before Michael Gregoritsch and Marcel Sabitzer added two in three minutes after the break.
It means Austria now top their Nations League group and they did it while playing some lovely stuff.
They also did it with a clean sheet, something that was hard to come by at Old Trafford in the second half of this campaign.
United kept six clean sheets in six months under Rangnick and fans think it's an indictment of their current squad.
One commented: "Just players willing to listen to instructions and work well. No coach was ever the problem at Manchester United."
Another said: "Ralf Rangnick is balling with a dead Austria team. The moment you realise the problem is United. The better for you."
A third put: "Rangnick is gonna do something mad like win the Nations League with Austria now, isn't he?"
While someone tweeted: "Good things happen when you leave Manchester United."
United fans were split on Rangnick by the time his spell in charge came to an end.
Many enjoyed his blunt but honest press conferences, which he used to slam the club's structure and failings over the last decade.
Some believed he could've been doing more on the pitch however, with the Red Devils scraping a sixth place finish in the Premier League.
United's final points total of 58 is their lowest in Premier League history too.
Rangnick was upbeat about his side's performance out in Osijek but admitted he wanted more goals.
The former RB Leipzig boss said: "We had problems in the first 25 minutes, but getting the lead just before the break did us good. In the second half we played really well and scored two wonderful goals.
"We could even have scored more. I am very satisfied."