Seidl refused to be drawn on the details of Sainz's deal but believes the regulations that state no current drivers can be involved in the December 15 test at the Yas Marina circuit automatically preclude him from doing so, despite the FIA allowing for "exceptions".
"First of all we never speak in public about the details of our contracts with our drivers," insisted Seidl. "But the regulations are clear so I don't see any way that Carlos is driving the car.
"There is no room for him to drive for Ferrari because unfortunately for him the rules do not allow. It's a young driver test.
"I only know what the rules are saying and in the end, if someone decides it's different, there's nothing I can do against it, so that's why there's no point wasting any more valuable energy we need.
"It's more a matter of principle, the entire discussion. It's not about Fernando driving or not, and again I focus on what we can influence here at McLaren, and that's becoming a better team and beating the competitors on track."
Seidl sees no point in clarifying matters with the FIA given the rules were laid down earlier in the year.
"For us, it's clear that based on the regulations that exist we can't run an active Formula 1 driver," added Seidl.
"I don't think it needs further clarification because there was a unique rule for this year which all teams agreed on in April, together with the FIA and Formula 1 when the pandemic was kicking in, in order to save costs for all teams involved.
"It was a package decided, along with skipping one test in winter testing. That was a unique situation for this year. As far as I know, the regulations are more clear again for next year and back to normal."
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