MANCHESTER, England, Dec 7 (Reuters) - Solving Manchester United's dreadful defending at set pieces has quickly become one of the main priorities for new head coach Ruben Amorim, whose best-laid tactical plans have been left in ruins due to his side's failure to clear the danger at corners.

Nottingham Forest needed just 90 seconds to score from a corner at Old Trafford on Saturday as United suffered a 3-2 loss and showed Amorim's side had learned little from a 2-0 midweek defeat at Arsenal when both goals came from that source.

"When this happens it's my fault, because I'm responsible. We tried to do it in the best way, we did a lot of work in this area because we saw it in (the) Arsenal (game), and it is really hard," Amorim told a news conference.

"In the beginning of the game (against Forest), you suffer on the first play, it's really hard then to put energy in the team, but we managed to draw the game, and then in the second half the same thing."

United's lack of height and power at the back is exposed at free kicks and corners and Amorim rejigged his team in the second half, bringing on centre-back Harry Maguire at 3-2 down in a sign that he was worried about conceding again.

"We put Harry Maguire on because of set pieces, to control more who was winning all the first balls. The only way to attack Nottingham was transitions, to try and win first and second balls. When Harry was on the pitch he won all the first balls," Amorim said.

However, by then the damage had already been done.

A simple screen had freed up Forest defender Nikola Milenkovic, who brushed off Lisandro Martinez to score.

Martinez and goalkeeper Andre Onana were then both at fault for Forest's third goal in the 54th minute, a looping header form Chris Wood that one of them should have dealt with.

"Everybody know the emphasis on set-pieces these days, it's huge, a big avenue for and against. We did well today," delighted New Zealand striker Wood said.

Amorim insisted he would stick to his principles, despite his shaky start at Old Trafford.

"This happens with a lot of clubs and we have to manage to continue doing the same things in improving the team, because this will turn around," he said.