The reason is because van Gaal told him he wasn't going to get any game time here... How's that going to change? If anything with the signing of new players last summer and the inevitable signings we make this coming summer, playing time will be even more at a premium.
And why he told him that?
"Listen Mr. Luis, you've been pretty crap last couple of seasons, I don't know why and I don't care. You've also been the most shit player in a preseason, so, you go to Lisbon and be good there while we're paying your salaries for the sake of it. If you perform well, we shall sell you to the club who offers 12,51m quid and not to that who offers 12,49m. Be well and write sometimes."
I don't think so. I get what you're saying but it's a bit deeper than just that IMO.
Going by this: "We also think about the scouting of our own players because we have a lot on own and then we have to analyse their performances", and their performances, Nani's going nowhere, Chicha and Tom are gone. Fine by me.