United host Anderlecht in Thursday's last-eight second leg having drawn 1-1 in Belgium last week.
Klopp told Sports 1 magazine: “Manchester United have a tremendously well-assembled squad.
“They are very likely to reach the Europa League final.
“We'll then have to see how many key players they still have available.
“They have already got injury problems.
“But with their extensive squad, they have been able to react to pretty much everything that has come their way.
“Recently they have had four or five men out injured and Henrikh Mkhitaryan still hasn't been able to break into their starting line-up.
“You can tell from that just how much quality they possess from the outset.
“United have a mix of experience, pace and quality. They are truly a good team.
“And to my mind they are the outright favourites to win the Europa League."