The Red Devils have stumbled into the new campaign, with a fortunate home win against Wolves followed by a disappointing drop-off in last weekend’s defeat at Tottenham.
Ten Hag accused some of his players of not doing their jobs in the capital and will have been furious by the meek start on Saturday that allowed Forest to race into a two-goal lead.
Fernandes came under particular criticism after Spurs and led United’s turnaround, scoring the match-winning penalty shortly after drawing the foul that saw Joe Worrall sent off.
Casemiro wasted a glorious chance to level but made amends shortly after half-time when turning home at the end of a clever free-kick routine.
Worrall’s dismissal for denying Fernandes a goalscoring opportunity gave United extra pep in their step, with the skipper soon scoring from the winner from the spot after Danilo was adjudged to have brought down Rashford.
They were among three changes against a Forest side who flew out of the block and launched a blistering counter-attack.
From a corner that followed an early Antony shot, Steve Cooper’s men cleared and Morgan Gibbs-White headed on. Awoniyi showed more hunger and strength than Rashford to win the ball inside his own half, roaring forwards and keeping his cool under pressure as he got close to goal, sitting down Andre Onana before prodding past him.
It was a fantastic Forest start that got even better in the fourth minute. Gibbs-White sent in a free-kick from the right and seemingly unsighted Boly turned home with a headed attempt that beat Onana and stunned Old Trafford.
"You’re getting sacked in the morning" chanted the Forest fans as the Old Trafford faithful called for their owners to sell up.
United looked punch drunk but eventually regained a modicum of composure, reducing the deficit in the 17th minute.
Fernandes’ long-ranger was stopped and Rashford eventually burst down the left past Serge Aurier and into space before driving across for Eriksen to flick home.
United pushed for a quickfire leveller and Casemiro somehow failed to level in the 26th minute. A corner came in from the right and bounced up for the midfielder, who inexplicably headed wide of an open goal from six yards.
The hosts continued to push but there were chances at the other end.
Gibbs-White was looking lively in front of England boss Gareth Southgate and saw a fizzing stoppage-time effort on goal inadvertently blocked by Awoniyi.
United started the second half far better than the first and equalised in the 52nd minute from a clever free-kick routine.
Forest were caught napping when Fernandes played over to Rashford, whose clipped cross was glanced back across goal by the Portuguese for Casemiro to score.
Old Trafford erupted and it was so nearly immediately 3-2, with Antony cutting onto his favoured left foot and whipping a beautiful strike that Matt Turner did well to stop.
Fernandes sent the Brazilian’s cutback just across the face of goal as United pushed for a winner and was never far away from the action.
The skipper drew a foul from counterpart Worrall in the 67th minute and referee Stuart Attwell brandished a red card having adjudged to him have denied a goalscoring opportunity.
The VAR ratified the decision, just as he did when Danilo caught Rashford in the box.
Fernandes stepped up to lash home low past Turner in the 76th minute as "Bruno, Bruno," echoed around a rocking Old Trafford.
Forest were now a man and a goal down, but they continued to push. Onana acrobatically denied Boly but United shut up shop, with players leaving to cheers and anti-Glazer chants.