Nani returned to boyhood club Sporting on a season-long loan in August as Argentina international defender Marcos Rojo moved in the opposite direction in a permanent deal.
The 28-year-old Portugal international found himself down the pecking order at Old Trafford after consecutive seasons of inconsistency in Manchester.
Nani's hopes of a new start under manager Louis van Gaal were dashed when Argentinean star Angel di Maria was prised away from Real Madrid in a £59.7 million transfer.
And after returning to Old Trafford for Tuesday's friendly against Argentina, Nani said his return to Portugal was necessary.
"I saw the opportunities at United were going to be less for me so I had to make a choice," he said.
"I had been at Old Trafford for a long time, for seven years, and I think I needed a little time away to understand and find my confidence again.
"Louis van Gaal gave me the option to go. I could have stayed but I chose the best option for me at the time and I think I was right."
Nani has flourished since moving back to Lisbon, scoring five goals in all competitions for the Portuguese outfit.
"I am very happy how it is going at Sporting. That is what I wanted. I wanted to play more. I wanted to get back to my best and I wanted to enjoy my football," he said.
"That is what is happening. It is going well. It has turned out very good for me. It has made me very confident again for the future. Yes I had to get away from United to achieve this.
"I am enjoying my game again. Football is the best thing in my life. Football has given me everything and I when I play I really enjoy myself so I want to play all the time. That wasn't happening when I was at United. When I don't play I am very disappointed.
"You can accept not playing regularly for one or two years but after that when you know you have the quality to play you have to make a choice. That is what I did and it is working out well."
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