Louis van Gaal explained Manchester United took inspiration from its UEFA Europa League opponent, FC Midtjylland, for its second goal in the 3-0 FA Cup win at Shrewsbury Town.

During his side's Europa League loss to the Danish club, Van Gaal noted its 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 after 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 learned 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.

Fullback Cameron Borthwick-Jackson made way at halftime 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 halftime because he was injured," the Dutchman said. "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 Friday's (AEDT) second leg against Midtjylland at Old Trafford, describing his knee injury as "not so heavy."