Interim coach Castro had overseen victories over Arouca and Europa League opponents Napoli since taking over from Paulo Fonseca earlier this month.
However, Islam Slimani's 52nd-minute header proved enough to condemn Porto to a fifth Primeira Liga defeat of the campaign.
Slimani pulled away from his marker at the far post to nod home Andre Martins' cross from the right.
Second-placed Sporting now hold a five-point lead over Porto and are four behind Benfica ahead of the leaders' match at Nacional on Monday.
At the bottom, the weekend's big winners were Pacos de Ferreira, who climbed out of the relegation zone with victory on Saturday before seeing the two clubs they leapfrogged lose on Sunday.
Basement outfit Olhanense are now winless in 13 away league games after a 2-0 reverse at the hands of Arouca.
Roberto Rodrigo and Lassad Nouioui were on target for Arouca, the latter sealing victory with an injury-time penalty.
Arouca, who saw Cristian Ceballos sent off late on, had previously gone seven Primeira Liga matches without a win, but can breathe a little easier after pulling five points clear of the bottom two.
Belenenses are alongside Olhanense on 17 points following a 3-1 loss at home to Vitoria Setubal.
Rafael Martins put the visitors in front just after the half-hour mark, only for Tiago Caeiro to level on 66 minutes.
Setubal then seized control in the closing stages with goals from Ricardo Horta and Martins.
