Barcelona superstar Messi is expected to play a part at the Wanda Metropolitano after being rested for the 2-0 win over Italy last week.
Should the five-time Ballon d'Or winner be given the nod, Koke acknowledges that it will be a very tough night for Spain against the player he considers to be the best in the world.
"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," he told Marca. "For me, he's the best in the world. When you're on the pitch against him, what you're thinking is that he doesn't have his best 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."