The sunbaked pitch was quick to provide action, with Luke Shaw taking one touch then firing the ball into the bottom corner to end his lengthy goal drought in the fifth minute.
Manchester stopper Senne Lammens denied a superlative strike from Morgan Gibbs-White after a defensive calamity from the hosts that went unpunished midway through the opening stanza.
Goals were fast and furious in the second half. Elliot Anderson planted a perfect delivery onto the head of Morato, who headed home his first Forest goal for the equaliser.
But 120 seconds later, Matheus Cunha pounced on a loose ball inside the area to snatch back the initiative for the home side. However, the goal wasn't without controversy, as Bryan Mbeumo looked to have touched the ball with his arm before his blocked shot fell to the Brazilian.
The twists and turns continued. United's Bruno Fernandes notched up a record-equalling 20th assist of the campaign, finding that man Mbuemo, who produced a smart finish to put Michael Carrick's men 3-2 up.
But Forest bounced back. An attack down the far side of the park saw Anderson fire in another cross, which found Gibbs-White's left foot as he opened up his body and side-footed the ball past a hapless Lammens.
Old Trafford then came to a standstill with Casmiro's departure after four seasons with the Red Devils.
The entertaining affair continued to ebb and flow with sparkling attacking football, with a brilliant save from Forest Matz Sels denying Joshua Zirkzee at the death.