Liverpool remained on the back foot after the interval and on the hour they fell behind.
Rooney picked out Darren Fletcher on the right and his tempting cross was met by Park whose diving header gave Reina no chance.
The Reds responded brightly to that setback and could have levelled inside a minute.
Gerrard burst forward and his low centre teed up Torres but for once the Spaniard's first touch let him down and the chance went begging.
Gary Neville was lucky to escape punishment after his boot caught Rodriguez in the head and then Darren Fletcher somehow got away with a talking to after lashing out at Dirk Kuyt.
Benitez rung the changes in a bid to find a way back into the contest with Alberto Aquilani and Babel coming on for Kuyt and Rodriguez.
The Reds started to enjoy more possession but a number of promising attacks broke down and they failed to test Van der Sar.
Yossi Benayoun replaced Lucas but as Liverpool committed more players forward United almost claimed a third goal.
The hosts counter-attacked swiftly with Rooney turning inside Agger before dragging a shot narrowly wide.
Liverpool could have equalised at the death but Torres scuffed his shot when well placed and Benayoun headed straight at the keeper.
Man Utd: Van der Sar, Neville, Ferdinand, Vidic, Evra, Valencia, Fletcher, Carrick, Park (Scholes 87), Nani (Giggs 79), Rooney.
Subs not used: Kuszczak, Berbatov, Rafael Da Silva, Jonathan Evans, Obertan.Liverpool: Reina, Johnson, Carragher, Agger, Insua, Mascherano, Lucas (Benayoun 83), Kuyt (Aquilani 73), Gerrard, Maxi (Babel 76), Torres.
Subs not used: Cavalieri, Kyrgiakos, Ngog, Kelly.
Referee: Howard Webb
Attendance: 75, 216
