Sunday's Argentina MotoGP was already looking unpredictable due to the absence of prior testing, rapidly improving lap times and threat of rain at the Termas de Rio Hondo circuit.
But the news that both rear tyres have been withdrawn and replaced by a new harder construction for race day, following a high-speed failure for Scott Redding in final practice, means an even greater leap into the unknown.
Pole sitter Marc Marquez said: “After this decision from Michelin, I think that tomorrow will be a bit of an unknown because everyone will have a different tyre spec from what we've used for the whole weekend. We'll wait and see how it works.
“We'll have two warm-up sessions, one of 30 minutes and the other 20, to gather information, and we'll have to see if everything goes well. Let's hope there won't be too many differences from the tyres we've been using so far.”
Repsol Honda team-mate Dani Pedrosa, who will start from the head of row two, added:
“Michelin has decided to change the rear tyre with one that we don't know. Clearly it's an unknown, and for this reason we'll have an extra session but most probably with less temperature in the asphalt comparing with when we will race.
“We don't know how it will work or which setting we'll choose. We can't say a lot until tomorrow, when we'll see how this new tyre works.”