Valentino Rossi and championship leader Fabio Quartararo admitted they are unsure why Monster Energy Yamaha are not utilising test rider Jorge Lorenzo more regularly.
Lorenzo retired last year after winning three MotoGP championships, the last of which came in 2015, before being signed up by the factory Yamaha team in January as their test rider.
However, while Red Bull KTM Factory Racing, Repsol Honda, Ducati and Aprilia have been carrying out tests in recent months, Lorenzo has not ridden the YZR-M1 since February's Sepang test.
Ahead of this weekend's San Marino Grand Prix on the new track surface at Misano, Rossi questioned why the experienced rider has not been called upon.
"I have the same question as you," he said at a news conference on Thursday when asked why Lorenzo is not present. "Honestly, I don't know.
"It is true we don't understand because in the past we know the capacity of Jorge on the Yamaha, so I don't know why he is not riding. He did 20 laps in Sepang and that is it.
"When we see all the riders in June and we don't see the Yamaha here, I won't say it is a disadvantage, but it is good to have some comments [on] our bike with the new asphalt."
Quartararo, who holds a three-point lead on closest challenger Andrea Dovizioso at the top of the drivers' standings, echoed the views of Rossi and aimed criticism at the team.
"I have exactly the same question for Yamaha. I was very happy when I understand Jorge is our test rider because he is one of the best riders in the history of the M1," he said.
"I think he can help us a lot – we see what [Dani] Pedrosa did with KTM in one year, I think he gives them a lot of good advice.
"Yamaha has to trust in the programme and sincerely I don't know. Sometimes things happen at Yamaha that is difficult to explain, so you have to speak with them.
"From tomorrow we will have to start from zero with the new asphalt and understand a lot of teams like KTM and Ducati already did in the last test. It will be harder I think."
Quartararo has stumbled through the past three races, accumulating just 20 points in the process, but still leads the way at the summit.