United forward Rasmus Hojlund cancelled out Nikola Milenkovic's early goal but Morgan Gibbs-White and Chris Wood put the visitors in charge and they kept United at bay after Bruno Fernandes pulled one back.

Forest's first win at Old Trafford since 1994 lifts them to fifth in the Premier League table but United are languishing in 13th spot.

Amorim made a positive start to his career in the United dugout after replacing the sacked Erik ten Hag last month, drawing one and winning two of his first three games.

But he warned the "storm will come" for the struggling club, who have endured a torrid start to the season.

Reality is now starting to bite hard after a 2-0 defeat at Arsenal in midweek followed by Saturday's reverse at home.

"Tough game in a tough moment but we have to continue the job," a disappointed Amorim told the BBC.

"This is a long journey. We are improving in some aspects. We have to win games to help us improve the team.

"We are in the start of something so big that we pay attention to every detail. We already know it's a big job so we have to continue."

The former Sporting Lisbon boss will likely be as frustrated with the manner of the goals as the result.

Set-piece agony 

Corners proved their undoing at Arsenal and Milenkovic headed home from an Elliot Anderson corner in the second minute after beating Lisandro Martinez to the ball.

United rallied and were level in the 18th minute with Hojlund turning home after Alejandro Garnacho's attempt from a Manuel Ugarte ball was smothered by Forest goalkeeper Matz Sels.

The home side failed to build on their goal and Forest's Jota Silva sent a header onto the crossbar after a free-kick was nodded on.

United captain Fernandes went close with a set-piece of his own as the first half wound down, hitting a 25-yard free-kick that Sels brilliantly got a fingertip to before it rattled the crossbar.

But Forest again made a lightning start to the second half.

Fernandes gave the ball away to Callum Hudson-Odoi and he found Gibbs-White, whose 20-yard strike swerved as Andre Onana was all too easily beaten.

New Zealand international Wood met a deep Gibbs-White cross in the 54th minute with a header back across goal that Matthijs de Ligt, Onana and Martinez all failed to deal with as it looped in.

United pulled one back just after the hour mark when Amad Diallo showed good skill and awareness after driving down the right to play back for Fernandes, who swept the ball past Sels from the edge of the box.

The goal injected new belief into the Old Trafford faithful, who roared with encouragement when Diallo went close soon afterwards.

But Forest saw out that surge and largely blunted the hosts. Diogo Dalot saw an effort saved and Diallo was denied by a block.

Substitute Marcus Rashford's volley deflected wide deep in stoppage time, with Martinez sending an acrobatic effort inches over before the final whistle.