Mandzukic is the tournament's joint top-scorer with three goals and Modric sparkled in the Croats' 3-1 win over Ireland in their opening game and the 1-1 comeback draw with Italy.
"Croatia are playing very well and many of their fans have come to Poland and that gives them a lift that makes them more dangerous," Del Bosque's assistant Paco Jimenez said on the Spanish football federation (RFEF) website.
"They have good players like Modric and Mandzukic and in defence it has worked out well for them putting [Vedran] Corluka in central defence," added Jimenez, who puts together the short films of opponents that the players study before matches.
"Modric has impressed me a great deal, he is a fantastic footballer and Mandzukic is playing very well.
"He gets up well [to head the ball] he fights for everything and we are going to have trouble controlling him effectively."
Holders Spain and Croatia top the group on four points ahead of Italy on two and eliminated Ireland on none.
Even if the Italians beat the Irish they would be knocked out as long as the other game is a scoring draw of two goals each or more.
Jimenez said he was spending Friday collating all the information he could find on Croatia to present to Del Bosque and the coach's number two Toni Grande.
"I used to do that at Real Madrid, I currently do it for the national team and after the tournament I will retire," he added.
"Now we enter all the data into the computer so we can access all the information together, because that makes things a bit faster now."
