Carlo Ancelotti branded Toni Kroos and Luka Modric as "untouchable" at Real Madrid, though did not confirm either midfielder will start against Barcelona.
Madrid head to Camp Nou on Wednesday for the second leg of a Copa del Rey semi-final tie that they trail 1-0 following Eder Militao's own goal in the reverse encounter last month.
Kroos, 33, and 37-year-old Modric both started that game, though the pair were substituted late in the second half.
While Kroos played the duration of Madrid's 6-0 win over Real Valladolid on Sunday, Modric was an unused substitute.
That would suggest Ancelotti was protecting the latter ahead of the third Clasico in the space of a month, but Madrid's coach declared neither of his veteran campaigners in midfield are guaranteed starters.
"Kroos and Modric are untouchable but that doesn't mean they're going to play tomorrow," Ancelotti told reporters.
"There are many untouchables but then you have to choose an eleven. I have to think about the resources I have on the bench.
"Starting isn't the most important thing, the important thing is to be effective [the game]."
While Kroos and Modric approach the twilight of their careers, Madrid are looking to build for the future with Aurelien Tchouameni and Eduardo Camavinga, who has been a consistent starter since the season resumed following the World Cup.
Asked about the importance of the physicality and athleticism that the France duo can provide, Ancelotti said: "If we talk about energy, it's clear that in these games it's important.
"An effective start from above can also give you an advantage."
However, he acknowledged "there's a lot of pressure in these games", adding that "the experience of veterans can be important".
With Madrid one of the clubs reportedly pushing to sign Borussia Dortmund star Jude Bellingham, Kroos and Modric could face uncertain futures.
They are both entering the final few months of their contracts, albeit renewals are said to be on the table.
"I see the players every day and they are very focused. It can be thought that when a contract ends it can affect them," Ancelotti said.
"It doesn't affect them and I don't have to tell them anything because they are very professional.
"Each one has to evaluate the role they have. It is the discussion that a player has to make and if they want to renew it is because they like the role they have."
Asked if he will discuss his team selection with any players left out of his starting XI, Ancelotti added: "Nothing special. I cannot talk to every single player who is not going to start.
"If they need to talk, my door is always open, but this group of players do not ask about explanations, they understand the situation, the moment.
"There are a lot of players who are very important, and the door will always be open to them."