Serie A: Sampdoria 1 Inter 0
The Brazilian-born forward was called into Antonio Conte's latest Italy squad on Saturday having gained citizenship, with his agent declaring the honour a "dream come true".
And 24 hours later, he was celebrating again, as he fired home a stunning free-kick to settle matters at the Stadio Luigi Ferraris.
After Inter had dominated the opening 20 minutes of the second half, hitting the woodwork through Mauro Icardi, Eder thumped in from 25 yards, leaving Inter goalkeeper Samir Handanovic with no chance.
In doing so, he made amends for his sending off in the same fixture last season, when he was dismissed as Inter won 4-0, as well as handing Sinisa Mihajlovic's side a fourth straight Serie A win.
Inter, meanwhile, are now winless in six matches across all competitions as Roberto Mancini - who enjoyed hero status during a 15-year stint as a player at Samp - saw his side fall to another defeat, three days after exiting the UEFA Europa League at the hands of Wolfsburg.
Sampdoria's Luis Muriel came into the game with three goals in five appearances since his arrival from Udinese, and he ought to have added to that tally in the 11th minute.
The Colombian turned away from Andrea Ranocchia with a superb piece of control only to blaze over the crossbar.
Inter's response was swift, as Icardi headed Xherdan Shaqiri's cross off target from six yards.
Eder then brought a comfortable stop out of Handanovic with a strike from distance and also fired over midway through the half.
The interval arrived without either goalkeeper having had to make a serious save, Muriel and Icardi both shooting over in the latter stages of the half.
Inter almost broke the deadlock in the opening minute of the second period when Icardi's curling effort struck the crossbar with Emiliano Viviano well beaten.
The Samp goalkeeper got down smartly to keep out Fredy Guarin's shot three minutes later and, from the resulting corner, Lukas Podolski failed to get a telling touch on Shaqiri's centre.
Inter were soon left to rue their wastefulness when, in the 65th minute, Eder was teed up by Samuel Eto'o and Angelo Palombo before hitting a shot that swerved away from Handanovic and went in off the right-hand post.
And although the visitors pressed for an equaliser in the closing stages, Samp held on to secure all three points and maintain their unbeaten home league record this season.