It started last week when an Instagram user claimed the FC Dallas midfielder cheated on the award-winning singer with a woman last year while at a nightclub in Madrid, Spain.

Along with the accusation, the person also seemingly provided receipts including alleged video, text messages, DMs and audio from Lletget.

FYI, Sebastian proposed to Becky G just 3 months ago.

After the posts went viral, the accuser's IG account was deleted ... but the damage had already been done. Days later, Lletget has decided to address the rumors (though not in specifics) in a lengthy statement, saying on Monday, "a 10 minute lapse in judgement results in an extortion plot."

"Since this person did not get what they wanted it has now become a public social media spectacle filled with more lies than any truth and false posts aimed at the love of my life, the one person who I should never take for granted or put at risk."

Lletget -- who joined the Dallas football club in 2022 -- apologized to Becky and vows to do better honoring their love.

"I know that any actions made that put us here should have never happened to begin with," Lletget said.

Sebastian continued ... "You have been the light in my life, my strength, who has always shown me unconditional love. Instead of honoring that love every day, I have done the opposite, hurting you, and disrespecting the one person I love more than anything."

"I am so sorry and Know I have to do whatever it takes to earn back the trust and love you deserve."

Lletget also revealed he'll participate in a wellness program to work on his mental health.