The Real Madrid midfielder, who was man of the match in Croatia's opening victory against Turkey, is expected to miss the final Group D encounter in Bordeaux after sustaining a groin injury in Friday's 2-2 draw with the Czech Republic.
"Modric is a great player," Del Bosque said in a news conference on Monday. "A lot of good plays go through him.
"He has had a great season and he is proving his quality with the national team. But Croatia are not just Modric.
"From what I've seen and how they have played their opening two games, Croatia are one of the best teams in the competition.
"They play at a high level and I expect a difficult game tomorrow."
Spain have already assured a last 16-place following their 1-0 triumph over the Czech Republic and Friday's 3-0 victory over Turkey.
Yet the holders still need a point to seal top spot and ensure that they are not involved in a rematch of the 2012 final with Italy, winners of Group E, in the round of 16 in Paris on June 27.
A draw would see Croatia progress to the next round in second place and have to face Italy next.
"I expect an open game," Del Bosque said. "We can't forget that we've been in this position before and just like in 2012, both teams will go out determined to get a win."
Spain were also paired against Croatia in Euro 2012 and needed a late Jesus Navas goal to beat them 1-0 and eliminate them from the competition en route to winning the title.
Not since their victorious 2008 European Championship have Spain started a major tournament by winning their opening game.
Del Bosque has started the same players for Spain's opening two games and reports in the Spanish media have claimed he will make a number of changes on Tuesday.
However, Del Bosque is keeping his cards close to his chest. "Our aim is to win this game and I will choose a team that will put us in a situation do just that," he said.
"We are playing very well at the back and we are also finding channels to break down defences," Del Bosque said.
(Writing by Adriana Garcia; Editing by Ian Chadband)
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