Messi and Argentina visit the Wanda Metropolitano for a friendly match with Spain on Tuesday, with the Barcelona star hoping to be involved after sitting out the 2-0 win over Italy on Friday with a hamstring problem.
Koke has had plenty of experience facing Messi at club level and he concedes there is little an opponent can do to prepare for the challenge the 30-year-old presents.
"For me, he's the best in the world," he told Marca . "When you're on the pitch against him, what you're thinking is that he doesn't have his best day.
"It's very difficult [to stop him]. You have to cover a lot of ground, always help in defence... and hope it's not his day.
"You know he's going to start a move, you see it, you prepare to cover it... and then he does it. Or, he'll suddenly invent something, because he has a lot of creativity.
"Why isn't he just taken out? Because it's almost impossible."
Argentina looked out of sorts for long spells against Italy, as Messi watched on from the Etihad Stadium stands, before second-half goals from Ever Banega and Manuel Lanzini sealed the win.
The performance prompted some critics to accuse Jorge Sampaoli's side of relying too much on their star number 10, but Koke says such dependence is hardly surprising.
"Everyone looks for him when it comes to games, either for Barcelona or his national team, because he decides matches with a goal, a pass," he said. "When you watch him on TV, it seems like he suddenly changes gear.
"If you have him on your team, it's normal that you'd look for him."
Koke believes they are beginning to win back some fans following disappointing campaigns at the 2014 finals and Euro 2016 in France.
"It's always a point of pride to wear the Spain shirt and to play in your own stadium is amazing," he added.
"The excitement around the national team has been lost a bit and we're recovering it with the good work we've done in the last two years. People have become engaged again."