The 25-year old Colombian striker, currently on loan with homeland club Millionarios, will soon complete his mooted move to Lisbon according to Sounders general manager Adrian Hanauer.
"This one is not a rumor but nothing is done (yet)," said Hanauer.
"We'll obviously announce something if and when it happens.
"The Portuguese league is a good league in Europe. Sporting has played Champions League somewhat regularly, lot of visibility in the league, so ... certainly he's not going to, no offense, the Swedish second division.
"I love Fredy as a player and a person, and he's been a very important part of the history of this club.
"I'm proud that he was a big part of the franchise and that he developed into a really good young man while he was here, because we want to win soccer games, develop soccer players and develop good young men, which, again, is why the door is always open (for Montero to return) - and you never know."