The 20 year-old would likely remain with the South American club on loan until the end of the season if a deal is completed.
Mac Allister himself revealed his delight as he confirmed his move to Brighton is close to being finalised.
"Just a moment ago my dad told me the deal is done," he told Radio Rivadavia on Monday. "I'm going to have to travel to undergo a medical and be back in time for next Monday.
"I wanted to stay here until the summer, luckily they will grant my wish and in June I'll travel to England. I'm looking forward to settling in the city and very happy to play in the Premier League."
Need a VPN? Or seeking to switch? Then support Tribal Football by clicking here to sign up with our official partners NordVPN. Not only will you receive the best VPN experience, but also great discounts by signing up via Tribal Football. Thanks for your support!