The Swede completed his move to United yesterday afternoon.
And he has penned an emotional message to the Portuguese giants on Instagram.
"Dear Benfica family, I would like to thank you from the bottom of my heart for the magnificent 5 years I spent with you.
"I put on my shirt with great pride and, hopefully, have conveyed to you in every game and every day my feeling.
"It was a great pleasure to play for you. No doubt you have a very special place in my heart.
"To all my colleagues and coaches that I have had over the years, thank you very much for your confidence and for the daily help that enabled me to become the man and the player that I am today.
"To all those who have helped me daily and to all who work in the Club, my great thanks. Thank you SL Benfica!"