Forest's first win at Old Trafford in 30 years moves them up to fifth in the standings on 25 points, while the scale facing the recently-appointed United coach was apparent as his side languish in 13th place on 19 points after a disjointed display that featured some glaring defensive errors.
Having conceded from two corners to Arsenal in a 2-0 defeat in midweek, United went behind from another corner within two minutes as Forest defender Nikola Milenkovic easily out-jumped Lisandro Martinez to head his first goal for the club.
United levelled in the 18th minute when Alejandro Garnacho’s initial shot was saved, but Danish striker Rasmus Hojlund pounced to slot home the rebound to ignite the home crowd.
The hosts started the second half equally badly and again they conceded within two minutes as Fernandes lost the ball. It was quickly fed to Morgan Gibbs-White, whose low bouncing drive somehow took Andre Onana by surprise and flew past him into the net.
Fernandes redeemed himself just after the hour mark, scoring with a superb shot from the edge of the box but despite being United’s best player for much of the game, he was replaced in the 76th minute by Mason Mount.
United continued to look disjointed until Marcus Rashford’s stoppage time volley was deflected behind for a corner, and Martinez sent another volley from the resulting set-piece whistling over the bar.
That was as close as they came as Forest stood firm to claim all three points.
Merseyside derby postponed, Man City continue to 'suffer'
Manchester City's struggles continued with a wobbly performance in a 2-2 Premier League draw at Crystal Palace, while Brentford extended their unbeaten home run with a 4-2 win over Newcastle United and Aston Villa beat Southampton 1-0.
Liverpool, who lead the standings, were set to kick off the weekend action at Everton but the last ever Merseyside league derby at Goodison Park was postponed due to storm Darragh.
Erling Haaland and Rico Lewis scored for City, who won in midweek but have lost four of their last six league games and twice trailed at a wet and windy Selhurst Park.
They remain fourth on 27 points, a point behind Chelsea and Arsenal and eight off Liverpool but having played a game more.
"We take a point. We fought incredible and came back twice," City manager Pep Guardiola told the BBC. "It's a season to suffer. We'll see what happens in the last months.
"We tried. We cannot talk about the title race when we lose four games in a row and draw. We have to do more games to be consistent. It's difficult in the situation we have. I'm really pleased for the players -- they fought and did everything."
BATTLING BRENTFORD
Brentford held on to beat Newcastle at the Gtech Community Stadium after the visitors twice came from a goal down in the first half before Nathan Collins and Kevin Schade struck to wrap up the points.
Brentford climbed to sixth place in increasingly windy conditions while the Magpies are 12th.
Bryan Mbeumo and Yoane Wissa also scored for Brentford, while Newcastle's goals came from Alexander Isak and Harvey Barnes.
"One of the best results you can have in that weather, both sides were struggling, the wind was carnage," Collins said.
Newcastle captain Bruno Guimaraes lashed out at his side after another disjointed display sent them home empty-handed.
"We have always been reactive, we need to concede a goal to score - we never score first, we concede a lot of goals, this cannot happen. We have to find a way to be consistent," he said.
DURAN STRIKE
A first-half strike by Aston Villa's Colombia forward Jhon Duran was enough to secure the points in their home win over bottom side Southampton.
"We have to thank our fans because it was difficult in the weather today," said Villa manager Unai Emery.
"They (Southampton) started the game well, first 20-25 minutes they had chances. Progressively in the second half we stopped them better.
"We were close to a second goal and until the last moment we were defending, trying to keep possession and trying to score another goal. At the end we deserved to win and a clean sheet too."
Duran had netted in the 24th minute, chasing down a long ball and capitalising on some sloppy defending before charging into the box and powering a finish past goalkeeper Joe Lumley.
"Fantastic goal and the action, fighting in the duel with the centre back and how he beat him and scored with a great shot," Emery said.
Villa moved up to fifth, while Southampton remain rooted to the basement and are eight points from the safety zone.
The postponed match in Liverpool was set to be the last Merseyside derby at Goodison, as Everton move from what has been their home since 1892 to their new stadium at season's end.
The decision was taken after a safety advisory group meeting attended by officials from both clubs plus representatives from Merseyside police and Liverpool city council and was because of the safety risk in the area due to strong winds.
