I really dont know if this is the right move for both United and Lindelöf. You have to understand that Lindelöf only has played in a starting role for Benfica since the middle of last season. He hasn't completed a whole season in a starting role yet. He was fantastic when Sweden won Euro under-21 2015 and last season when Benfica played in CL, but has not been any special in the national team. He has been playing, and been okay, but not in any way a world class defender. His position in Benfica is perfect for him, he gets to play in a decent level week after week, and also in CL every season. He can as a talent, which he still is, develop in his own pace, without to much pressure, to hopefully become a world class defender. But to go to United, where you complete with Jones, Bailly, Smalling and Rojo is risking everything if he doesn't get minutes on the pitch, which is definitely not certain he will. And for United, to not but a world class ready defender and only one who maybe will become one and definitely needs to play to be able to become one, is very risky. And ones again, Lindelöf has only played in a starting role in Benfica since the middle of last season.
Lindelöf is not a world class defender, he is a talent, and he can become one, but his chances to that is a lot bigger in Benfica than in United.