I don't think he would do all that badly at all. If he suddenly lost the side effect, it would probably be very disconcerting at the start, but he would be able to adapt (now, what that would mean for the wider Border that's a different discussion altogether) and I think remain a pretty effective fighter.

He tends to use his side effect more for laying (and avoiding) traps and big or sudden attacks, but when you're in the middle of a fight, the typical swordplay is all him. It's even stated that he tends to lose duels with Tachikawa because he's too busy peeking at the future to concentrate on the present, and it's why he needs his glasses. In a way, it's a burden, constantly flashing images of what could be while you're trying to focus on the sword that's about to skewer you. No wonder he needs the coloured glasses!

So yeah, I think he would still be a pretty terrifying attacker, especially as he's been practising for a good number of years. He would probably focus more on the attack than defence (in which he currently has 15 fricking points, what the hell) and need time to adjust to not having foresight, but I think he's been shown as an effective fighter even in spite of and not just because of his side effect.

An example is these unused panels: in these, Jin was the one to take down the Ilgar in what ended up Tachikawa's move in canon. Not really something you'd need to use a side effect for, but you would need to be a proficient attacker to pull it off. (Also, he just looks very cool here)

Although I could also totally see him pull a Fuyushima and decide to become a Trapper. I feel like he'd be super annoying over the comms, though.

What would Jin be like if he lost his side effect?