Dorgu delivers winner for depleted Man Utd against Newcastle
Shorn of captain Bruno Fernandes due to injury plus the absent Bryan Mbeumo and Amad Diallo due to the Africa Cup of Nations, Amorim's change of role for Dorgu paid dividends.
The Danish international connected sweetly with a looping ball on his left foot for the only goal to compound Newcastle's miserable away form despite the visitors dominating the second half.
A first home win in two months takes United level on points with fourth-placed Chelsea, while Newcastle remain down in 11th.
After a series of promising performances went without reward in recent weeks, this time United got over the line despite having to mount a rearguard action to secure just a second clean sheet of the season.
Amorim responded to the loss of Fernandes by finally switching from his preferred 3-4-3 system to a back four with Dorgu thrust into a more attacking role on the right side of a three behind Benjamin Sesko up front.
Dorgu duly delivered his first goal for United since joining from Lecce nearly a year ago.
Diogo Dalot's cross was cleared high into the air before the 21-year-old volleyed past Aaron Ramsdale on 24 minutes.
Newcastle boss Eddie Howe himself made an early formation change to adapt to a fast start from United.
But Howe has struggled all season for answers to the Magpies' woeful form on the road.
Newcastle have won just twice in 12 games away from Tyneside in all competitions.
Ramsdale denied Dorgu a second at the end of another dangerous United counter-attack before half-time.
Yet, Amorim's forward options were dealt another blow when Mason Mount did not reappear for the second half.
Jack Fletcher, son of former United midfielder Darren Fletcher, was introduced for his first appearance at Old Trafford.
Newcastle sensed their opportunity as they penned the home side inside their own half for practically the entire second half without reward.
Amorim tinkered his system once more to try and turn the tide, but his substitution of Casemiro left the Brazilian befuddled.
Despite barely being able to escape out their own half, United still had two huge chances to kill the game off.
Sesko smashed off the bar from a narrow angle before Dalot blazed over after being picked out by Lisandro Martinez's free-kick.
Lewis Hall crashed a strike off the crossbar with Newcastle's best effort.
Anthony Gordon and Lewis Miley both fired wastefully high over the bar after intricate play to finally create some space inside the United box.
But Newcastle joined local rivals Sunderland as the only sides to not score against United this season in the latest blow to their hopes of a return to the Champions League next year.