The former Cardiff forward suffered a broken skull in the vicious attack, his club Rapid Vienna have confirmed.
A statement from the club read: "There is some very bad news to report before SK Rapid's last home game of the season.
"Guido Burgstaller was the victim of a physical attack in the presence of witnesses at the weekend and suffered serious head injuries.
"The 35-year-old from Carinthia was attacked by an unknown man in the centre of Vienna and suffered a fractured skull after a fall caused by a brutal blow.
"Burgstaller was immediately examined and treated in a Viennese hospital. Unfortunately, he will have to spend the next few days there and it is also a fact that he will not be able to work for several months.
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"SK Rapid wishes its worthy player a speedy and uncomplicated recovery and trusts in the responsible authorities that the as yet unknown perpetrator will be brought to justice quickly.
"We ask that you respect the privacy of Guido Burgstaller and his family.
"If there is any news in the next few days, SK Rapid will report it in consultation with its medical department and Guido Burgstaller.
"SK Rapid will of course fully support Guido Burgstaller on his way to a full and hopefully speedy recovery."
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Burgstaller previously spent six months in the Championship with Cardiff after signing in the summer of 2014.
During that time, he scored once in five appearances for the Bluebirds.
The 35-year-old played for German sides Nuremberg, Schalke and St. Pauli before returning to Austria in 2022.
He penned a new deal with Rapid in the summer before announcing his plans to retire at the end of the current campaign.
In 24 appearances across all competitions this term, Burgstaller has scored eight goals and provided six assists.
Rapid are currently third in the Austrian Bundesliga, eight points behind league leaders Sturm Graz.
The club are next in action in the Europa Conference League against Copenhagen on Thursday.
Meanwhile, their league campaign will resume in February following the winter break.