The hosts went behind early on but a goal from the Portuguese in the second half was all Walter Mazzarri's men could muster at San Siro
Rolando, on loan from Porto, scored the equaliser early in the second half after Mauricio Pinilla had put Cagliari in front from the penalty spot late in the first half.
Despite creating the clearest chances and hitting the bar late on through Mauro Icardi, the Nerazzurri could not find the winning goal to make it three successive wins and Rolando's leveller left Cagliari still looking for their first away win in Serie A since last February.
It has been a week of upheaval for struggling Cagliari, who parted company with assistant coach Ivo Pulga earlier this week, but the visitors impressed early on at San Siro, with Victor Ibarbo in particular causing problems.
Inter went into the game unbeaten in nine at home in all competitions since their solitary San Siro loss this season to Roma and Jonathan had their first chance of note but sliced well wide inside the area.
Walter Samuel scored the only goal in Inter's last home game - a 1-0 win over Sassuolo - and almost repeated the trick after 23 minutes when he out-jumped two players to meet Ricky Alvarez's teasing cross, but the direction was lacking.
As the half wore on, the home side exerted their authority, but a lack of real class was evident. Zdravko Kuzmanovic found space 30 yards out and lined up a shot only to fire well wide.
But it was Cagliari who took the lead after Juan Jesus handled in the area after 40 minutes. Pinilla made no mistake with his penalty, firing into the right corner and sending Samir Handanovic the wrong way to notch his sixth goal of the season.
A disappointing half for Inter was almost rescued in injury-time but, although Diego Milito's fierce effort was spilled by Vlada Avramov, the goalkeeper held on at the second attempt before Alvarez could get to the rebound.
Inter coach Walter Mazzarri changed things up at half-time and brought on Icardi and his impact was instant, as he provided a lovely cushioned header across the box after 51 minutes for Rolando to sweep under Avramov from close range.
Cagliari were not deterred from their task and space opened up for Andrea Cossu soon after and although his shot did not hit the target, it had the sprawling Handanovic worried.
Parity was only maintained after a wonderful Avramov save after the hour mark. Fredy Guarin chipped a delightful ball to the onrushing Rodrigo Palacio, who volleyed from 10 yards. The effort looked destined to find the corner, only for Avramov's outstretched leg to deny the Argentine.
Inter went agonisingly close to snatching all three points in the final minute when Icardi beat Avramov to the ball, but the header cannoned back off the bar and Cagliari scrambled clear.