Fages is out of contract at the end of the season and, though champions Saints want to retain him, they know salary cap pressures mean that might not be possible.
He has featured in the last two Grand Final wins and become an integral part of their side.
Watson said: “Theo has been a top player for a number of years.
“He was at Salford when I was there so we’re definitely interested in top-quality players.
“But nothing has been done or sorted out on the back of that.
“There has been an interest lodged and when half-backs become available and of that kind of quality... and Theo is a homegrown half-back as well.
“He’s 26 and look at what he’s done at Saints. He controls and organises their team so I think every team in the country, if not the NRL, will be interested in Theo.
“We definitely have an interest and have registered that interest with his agent but you can’t ever just go off that.”
Giants’ ‘marquee’ signing and fellow No 7 Aidan Sezer is also off contract at the end of the season but Watson insists that any move for Fages is not dependent on the Australian leaving.
Meanwhile, he said young centre Jake Wardle is ready to return from injury for Monday’s trip to Warrington Wolves.
Watson said: “It’s huge to have him back and hopefully he can stay fit and have a great season.
“We’ve been a bit up and down with the availability of the centres, so it will be good to potentially get our first-choice centre pairing out there on Monday.”
However, Joe Wardle – his elder brother – has suffered a setback in his recovery from a hamstring injury so still awaits his first game of the year.
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