It has to be “defeated by” since the defeated party is the object. If you want to use “to” then it needs to be “lost to” because the losing party is the subject. Someone has mentioned “defeat against” but here the word “defeat” is a noun and not the past participle of a verb such as “defeated” so we would have something like “Manchester United suffered a defeat against Liverpool” which I think is OK, as would be the use of “by”.