The Reds resisted a fightback from the Seagulls to secure a third successive win, when Bryan Mbeumo bagged his second of the evening, with a fourth a thumping strike at the death.
Matheus Cunha and Casemiro were also on target, the first time two Brazilians have scored in a league game for United, ensuring efforts by Danny Welbeck and Charalampos Kostoulas proved in vain for Fabian Hurzeler's side.
During his post-match interviews with Sky Sports and MUTV, Ruben said he could tell his squad's attitude in the build-up to kick-off was more confident, and ready to secure another three points.