The 25-year-old has sat out Liverpool's last three games - all of which have ended in defeat - with a groin problem but marked his return to full fitness in style by lashing home in the 65th minute.
Antonio Valencia hit the bar in response as United threw men forward before Nemanja Vidic and Argentinian midfielder Javier Mascherano were given their marching orders late on for second bookable offences.
Substitute David N'Gog sealed all three points deep into injury time to end a run of 11 games without a defeat for Sir Alex Ferguson's troops and four without a win for Benitez.
Liverpool were able to call on Torres but not Steven Gerrard, who failed to recover from the groin injury he aggravated during Tuesday's Champions League defeat to Lyon.
Despite that United had keeper Edwin van der Sar to thank for keeping the scoreline goalless early on after he first saved Fabio Aurelio's curling free-kick then denied Dirk Kuyt from the rebound.
Dutchman Kuyt had another glorious chance less than ten minutes later combining with Lucas Leiva only to scuff his shot well wide of van der Sar's right post.
Manchester United slowly began imposing themselves on the game but only had a tame Wayne Rooney - like Torres returning from injury - header to show for it.
Liverpool were once again on top after the break but couldn't find the back of the net - their woes in front of goal typified by yet another Kuyt miss.
However, the deadlock was finally broken in the 65th minute when Yossi Benayoun fed Torres, who held off Rio Ferdinand with ease before firing home from deep inside United's box.
Tempers flared late on when Vidic saw red for the third straight match against Liverpool after holding back Kuyt and Mascherano followed suit for a lunge on van der Sar all before N'Gog's strike.