Llorente, who suffered a facial injury during training this week but should face Racing in tomorrow's northern derby at San Mames, is enjoying a superb season and his tally of 11 league goals is only four less than Racing have managed as a team.
The Spain striker has caused problems galore for opponents with his strength and power, and Portugal knows exactly what his side must contend with tomorrow.
However, he is also well aware that knowing what to expect from Athletic and Llorente, and then successfully dealing with it, are two very different things.
"They have quite a defined style of play, but even though you know what it is, it's not easy to stop players like Llorente," Portugal said.
"Their strong point is Llorente, he is their reference. They look to get crosses into the box because with Llorente and (Gaizka) Toquero they cause a lot of danger."
Nevertheless, Portugal is still hoping the Cantabrians can achieve his goal of reaching the midway point of the campaign with 23 points - which would mean winning at San Mames.
"I would like to finish the first half of the season with more than 22 points and now we have 20. So, there's still a chance," he added.
"It will be very difficult, we know that, but of course we are going to try and get the win."
Portugal also revealed what it would mean to his side's fans for Racing to down the Basque outfit, saying in El Correo: "In Cantabria they like to beat Athletic more than Real Madrid or Barcelona.
"It's a rivalry that is seen more from this side than from Bilbao. Let's see if we're capable of causing a surprise this time and come out winners."
Athletic, who are unbeaten in five clashes with Racing and won each of the last three meetings, go into tomorrow's match in ninth place in the standings with 26 points.
They needed a late equaliser to draw 1-1 with Malaga last time out, but that result means they are just one point behind sixth-placed Atletico Madrid and the European places.
Athletic's next two games are at home, and winger Susaeta said: "Winning these two matches at San Mames would give us a big boost in the table."