I dunno. I’d be real tempted to get a Cubs jersey and wear it 13 times a year - all of the games against the Cards. 
Other than that, I’d probably just keep them in the hunt.
Really not trying to be contradictory, here-- I clicked on this, because of the many possibilities of what a “Jersey” could be, and even that the way it was phrased made it sound like some long-standing unsolved question I didn’t know about.
I got stuck on “watch the game.” Other than a little baseball when I was a kid, sports are an anathema to me.
A “good faith effort” to watch the game would probably involve a lot of being distracted by my phone, a laptop, maybe playing something on Audible, while watching the game with the sound off.
Seriously.
But if I were given the Jersey, I wouldn’t just leave it hanging in the closet. I’d think of something.
I’m not sure placing a bet would occur to me, since I’ve never placed one in my life, and would be naive enough about odds and whatever I’ll probably be accused of something; however, what would occur to me is spoiling the sport enough to make the general public less interested in it.
My biggest problem would be picking a favorite team.
That’s an honest answer from a non-sports person, who thinks we can contribute too.
I’m a relatively spoiled sports fan - I’ve been through three championship runs/attempts for my three favorite teams in the last 10 years. The Chiefs have won 3 Super Bowls and been to 7 AFC title games in a row. The Royals went to 2 World Series in a row and won 1 of them. ND football has been to 2 national championships in 13 years and come up empty. (My wife is a UGA grad, so I’ve been tangentially connected to football championship fandom.)
I derived more fan-related joy from the Royals, so choose that for my jersey (name and number TBD). I honestly don’t know how I’d use it to affect the team over the course of a season (or series of seasons). I’d like to think that I’d only just use it for Game 1 of any playoff series to give them an edge, but I can’t guarantee I wouldn’t be suiting up for Game 7 or any other elimination game.
I would use it during the long regular season to try to take down the sports betting industry. I think it’s a massive stain on society and sports itself, and both would be improved if someone were able to place system-breaking parlays.
Well, the best source of information we have on magic is folklore, and in folklore, things like this always come at some dire price. But if we ignore that, then absent any information, the best we can assume is that the side effects are random, and random changes still usually end up being for the worse.
Or, just to stick to the one thing we do know the jersey does, and even ignoring side effects: We know it makes your team win games, but we don’t know how. Does it make your team play better, or does it make the other team play worse? Maybe the way it works is by causing serious injuries to the other team. Do I want to hurt people for money?
I’d get rich then put it away.
After a night’s contemplation I think I’d a white with navy hooped Geelong jersey ideally with “G.Ablett Snr is god” on the back. Will display it behind glass on office wall. Won’t wear it … purely a defensive strategy to ensure no bloody Collingwood, Carlton, Hawthorn or Essendon supporting pillock get their grubby hands on it.
Are you speaking Football (AFL) or Soccer (FIFA), all I’m sure about is Oz.
Australian Football League (AFL) only, thank you.
Other codes can consult their own moral compass.
I would need a time machine to go along with the jersey. So I could put it work for the Washington Senators.
I saw that movie (and the play for that matter), pretty good movie.
Monetization is the answer. Start with just regular bets, spread them around enough to both make money and avoid suspicion. But once you’ve got a nest egg, go for a few proposition bets to make bank. “The Piggers are going to win every road game against Eastern Division teams and make it to the playoffs.” Something that makes sense as a fan enthusiasm thing, but will have good payouts.
Yep, that’s my answer. Why watch the game at all if you know who’s going to win?
Assuming you’ve bet on the game, it’s easy work if you can get it. And I’d be interested in how they win. I’d be watching for highly improbable events.
You are clearly not CEO material.
A potentially interesting wrinkle on this idea might be that the Jersey only works once per calendar year. Choose wisely!
That’s one of the factors I thought made this scenario fascinating.
On the other hand, although it would ruin the sporting interest for you personally, it would be fun to see how fans, media and statisticians would react to an MLB team going on a 10,000 game winning streak! Would people just stop watching at some point? Would the team be banned for cheating, even though nobody could figure out what they were doing?
I would take disabled/sick kids to watch their favorite team win.
After making a Butt-load of money gambling.
It would definitely be entertaining to see the sports pundits - and even the team players and coaches themselves - be unable to figure out what’s happening, and to be the only person in the know.
I’d have to think about whether I support my province in rugby, who are pretty much perennial winners and have low odds, or choose one of the easy beats for better odds and making more money.
I wouldn’t want a lot - may $4 or $5k at the start of the season to win the grand final when the odds are highest which should give a $60k plus payoff.
Then some smaller $1,000 bets per game would give me enough to live comfortably.