This is a great buy for United and at a reasonable price, given today's prices. I'm Portuguese and, above all, a huge Benfica fan.
We are sad to see him go but unfortunately we are used to losing our best players (especially the younger ones) after a brief period of time.
Having said that, United is my 2nd team (I'd say that I like United more than your casual fan from overseas) and I'm fairly convinced that Lindelöf will impose himself quite easily and quickly (like Bailly if it weren't for the injuries and African Cup of Nations).
We desperately needed a centre back and I doubt there's a better one in the market, factoring age, current ability, potential & price.
He reads the game superbly for a 22-year old and he's typical Scandinavian when it comes to calmness and concentration. Errors and almost inexistent, unlike Jones, Smalling and Blind. He's fairly fast (not a speedster, mind), strong and very difficult on one-on-ones.
And he's commanding. He only started playing regularly in the league on January 31st, 2016, because of injuries to both Luisão (already injured for some time at that point) and Lisandro López (during a game). He was subbed in on that day, partnering Jardel, and never left teh starting lineup again, even when (club captain and legend) Luisão came back from his injury. By mid-February he had already played FC Porto and Zenit, then away games at Sporting and Zenit (partnering the adapted DM Samaris) in the beginning March and, by that time, he was already commanding the defensive line. This season, Luisão came back to the fore and it was Jardel who was dropped.
One of his best qualities is he's very good on the ball,. If the opponents are marking the DM and leave him with space, he will for sure carry the ball (and the team) forward and he's great at finding (with quality) the RB or LB in advanced positions.
The only thing he's still lacking is assertiveness on dead ball situations, both defensively and offensively. Still a lot to improve there.
Bonus: he scored a peach of a direct free kick at Sporting in April (matchday 30 out of 34), earning Benfica a draw which kept us in the top-spot and proved out to be essential to our "TETRA" (4 league titles in a row). Never had even attempted one before (at least in the first team)... Don't know if he's going to keep working on that, but that was a helluva goal, when it mattered the most.