Although United and Fenerbahce have yet to agree a fee for Van Persie, the Dutchman signed a three-year contract with the Turkish club at an unveiling ceremony on Tuesday.
The move came as something of a surprise given Van Persie was Netherlands captain under the 63-year-old at the World Cup just 12 months ago.
But Van Gaal once again exhibited his ruthless streak by deciding to sell the 31-year-old after he scored just 10 goals for the club last term.
Van Gaal said: “I had the conversation at the end of the season, so he knew already what was coming. I told him in a conversation with him and we have to keep it like that.
“He knew that from the day we played golf, we spoke about the prospects of him at United. Every decision is difficult to sign and let them go.”
The former Barcelona coach believes Van Persie will feel right at home in Istanbul.
He said: “I think he did a very good choice. When I was (Netherlands) coach, I watched Fenerbahce because of (Dirk) Kuyt and I was always received in a very warm way.
“I think Robin will like it, to be in that warm club. I wish him all the best and I have also texted that to him.”