The 1-2 loss against in-form Sirius at home at Parken was the fifth in a row in the series and the crisis club is living dangerously with only one match left to play.
Degerfors and Öster got new hope with Sirius' clear victory.
For Degerfors, who are in the qualifying place, the gap to Norrköping is still three points but the club has two games left to play. On Monday, Hammarby awaits at home at Stora Valla.
Öster, in the relegation zone and four points behind Norrköping, has Gais away on Sunday.
Norrköping resisted during the first half. Four minutes after the break, the first setback came. Joakim Persson easily won the header duel with Marcus Baggesen on Oscar Krusnell's left cross.
Walta showed class
Leo Walta dealt the next blow.
The Finn – one of the hottest Allsvenskan players in one of the teams of the fall – scored 2–0 with his left foot and his 16th goal of the season after a lightning-fast counterattack.
"Over 90 minutes, we are good," says Sirius defender and team captain Henrik Rönnlöf Castegren, formerly of IFK Norrköping, to HBO Max.
In added time, Tim Prica reduced and gave Norrköping contact, but it was too late.
The desperation at the end is the stuff we need. When we pull out our hearts. That desperation has to be there from the first fucking second, says Norrköping coach Martin Falk to HBO Max.
Norrköping will finish the Allsvenskan against IFK Göteborg at Gamla Ullevi on Sunday next week.
We still have everything in our own hands, we will have a good match and win there, says Falk.
