The Italian club will play in Serie A next season after an 11-year stay following 1-0 loss to US Citta di Palermo at the weekend.
Franco Vazquez’s strike from just inside the box in the ninth minute proved enough to seal Cagliari's fate.
Nainggolan played for Cagliari from 2010 to 2014, when he moved to Serie A outfit Roma.
The Belgium international seems sad to see his former club go down to the second tier of Italian football, taking to Twitter to post the following message, which when translated from Italian reads: “I regret that Cagliari are no longer in the top flight… But I believe and I hope that they will return to Serie A soon.”
Mi dispiace che il @CagliariCalcio nn rimane nella massima serie... Ma credo e spero che ritornera prima possibile in serie a...— Radja Nainggolan (@OfficialRadja) May 18, 2015
Leeds United link
Cagliari is the former club of Leeds United’s current owner Massimo Cellino. The Italian entrepreneur sold the Sardinian outfit to an Italian company Fluorsid Group for £36 million in June 2014, according to The Guardian.
Cellino purchased Cagliari in 1992 and sold them after buying a 75% stake in Leeds in 2014.
Nainggolan actually played at Cagliari during the final years of Cellino’s reign at the club.
Manchester United link
Nainggolan has been linked with a move to Premier League giants Manchester United in the summer transfer window.
In 33 Serie A appearances this season, Nainggolan has scored five goals and created 35 chances, while having a pass accuracy of 87% and a shot accuracy of 45%.