United and Sevilla meet in Cologne on Sunday for the right to play against either Inter of Shakhtar Donetsk in the final.
Sevilla come into the match on the back of a 19-game unbeaten run, boosted by wins over Roma and Wolves in their past two outings, while United have lost just once since January.
While United were underwhelming in their 1-0 quarter-final win over Copenhagen, requiring extra time to emerge victorious, Lopetegui clearly rates Ole Gunnar Solskjaer's men highly.
Sevilla stunned United in the last 16 of the Champions League when they last met two years ago, but Lopetegui is certain the Red Devils are much better now.
"This United is the best Manchester United in recent years, a team that has found its way, that's why they haven't lost in the Premier League since January," he said.
"They have found the space for the many quality footballers that they have. They are complete, they will force us to play a great game to compete with them.
"This is what I hope for tomorrow, a semi-final in a game of maximum difficulty and that will require us at all levels to play collectively.
"We like to play beautiful and important matches. For that we have fought and worked all year. It is an important game against a great opponent.
"We will be able to arrive prepared. We will try to overcome them, no matter how big a team they are."
Sevilla are chasing their sixth title in Europe's secondary competition, having won the Europa League three times and its predecessor – the UEFA Cup – twice.