Idris pulled off a try-saving tackle on Pearce and then a bone-jarring hit on back-rower Boyd Cordner that forced a knock-on as the teams went to the halftime break locked at 6-6.
But his best hit came early in the second half when he smashed Roosters skipper Braith Anasta in the in-goal area to force a line drop-out in front of 11,478 boisterous fans.
It was no coincidence that the Titans conceded two tries - to Kenny-Dowall and fullback Anthony Minichiello - when Idris was off the field getting treatment on his sore hamstring.
Those tries allowed the visitors to jump out to an 18-6 lead after 56 minutes against a Titans outfit missing international forward Greg Bird (ankle) and hooker Kayne Lawton (hamstring).
Winger David Mead, in his return from a broken jaw suffered in round two, scored the second of his two tries in the 71st minute to keep the Titans in the match.
The home side kept attacking in the dying stages but, in a tense finish, the Roosters held on grimly to notch their fourth win of the season.