Louis van Gaal explained Manchester United took inspiration from Europa League opponents Midtjylland for their second goal in the 3-0 FA Cup win at Shrewsbury Town.
During Thursday's Europa League loss to the Danish club, Van Gaal noted their players dashing back from offside positions before free-kicks were taken and complained to the officials.
After being told the move was legal, the United boss stationed a cluster of his men behind the Shrewsbury defensive wall during first-half stoppage time at Greenhous Meadow, with Juan Mata standing over a free-kick and the fifth-round tie poised at 1-0 following Chris Smalling's opener.
Mata dispatched a crisp effort as Jesse Lingard, Ander Herrera and Anthony Martial charged towards the ball, although replays showed they were not back onside before the Spain international struck the ball
"We have learnt because we have analysed Midtjylland and we have seen that they are doing that," Van Gaal told BT Sport, after watching Lingard complete the scoring and secure a home quarter-final against West Ham.
"We think that it is always offside. We have asked the Portuguese referees and they said 'no, no - you can do that'.
"So we learned that off Midtjylland and have scored the free-kick. That is good, I think."
Van Gaal named a strong starting XI to guard against another mishap and further pressure on his increasingly unpopular Old Trafford tenure, although his injury problems mounted.
Full-back Cameron Borthwick-Jackson made way at half-time and substitute forward Will Keane limped out of the action with a groin problem.
"This is unbelievable because also Cameron Borthwick-Jackson, I changed him at half-time because he was injured," the Dutchman explained.
"I changed Mata to give him rest and then also Martial, and the substitute [Keane] is then also injured. It is unbelievable."
Van Gaal did not rule out goalkeeper David de Gea making a return in Thursday's second leg against Midtjylland at Old Trafford, describing his knee injury as "not so heavy".
With Champions League qualification on the table for the winners of the Europa League, the United boss insisted it is vital his team turn around a 2-1 deficit in three days' time.
"It is our chance to qualify [for the Champions League] ourselves, [even if] we have to win a lot of games then," he added. "That is why it is so important."