Mitroglou – a Greece international – was signed in January 2014 for a club record £12m fee from Olympiakos, but has only played three times for the Whites.
He returned to his former club on a season-long loan last term and Symons says he is happy for the player to join another club.
Symons confirmed: “There’s a good chance that Kostas will be off on loan shortly. If so it will be to Benfica but it’s not confirmed yet.
“It’s not disappointing. All along he’s been open and straight and said he wanted to play for Fulham if we were in the Premier League. He’s said all along he doesn’t want to play in the Championship.
“He’s enjoyed himself on the training camp this summer and hopefully he will get his wish, we’ll get promoted, and he can play for Fulham in the Premier League next year.”