Having plundered 22 goals during his first season in British football last season, Michu has started the new campaign in similar form, having already netted five times in all competitions.
His stunning form since joining from Rayo Vallecano has seen La Roja boss Vicente del Bosque come over to watch the 27-year-old in action, and he praised the forward in a recent interview.
Defender Amat played against Michu in La Liga, and having joined him at Swansea, the Spain Under-21 cap has no doubt he has the quality to be part of a squad including the likes of Andres Iniesta, Xavi and Alvaro Negredo, who hold the World Cup and European Championship titles.
"Rayo Vallecano are a small club in Spain and Michu was probably their best player," he said.
"The people in Spain are not surprised at how he has done in England because they know Michu is a good players who scored lots of goals in the first and second division.
"I think here he's a star. It's possible he could get a call-up to the Spain squad, and some players in the under-21 side have asked me about this.
"He has the level, and the level of the national team in Spain is very, very high.
"I don't know when, but it's possible he'll get a call up."
Amat will continue at the heart of the Swansea defence this evening with Ashley Williams again sidelined by an ankle ligament injury.
The Swans started their Group A campaign in magnificent style with a 3-0 humbling of Valencia at the Mestalla.
Amat knows how vital it is to build on that result against a St Gallen side who also won their opening fixture, against Kuban Krasnodar.
"It's a different game," he said. "Valencia was a very good win, but every game is different and I think we have to follow that win with another win.
"It's difficult to know a lot about them because it's a different league, but with the manager and the videos we have watched we will know a lot.
"The important thing is my team, Swansea and what we can do.
"It's a very difficult game because they also won their first game and it's important for us to win our first game at home."