Seven-time MotoGP champion Rossi is yet to record a win this season and sits 87 points behind championship leader Marquez with five races remaining.
He is the highest-placed Yamaha rider, though, and had hoped the 2019 engine would enable him to close the gap in the standings.
However, the Italian was not impressed with his first test of next season's offering.
Are we seeing the best of @ValeYellow46 despite Yamaha's struggles?
Iwata factory are in a worrying phase but nevertheless 'The Doctor' remains third in the Championship, and that's not all...#MotoGP | #AragonGP https://t.co/zeWJT0QA7l pic.twitter.com/NEFIIuRG1o
— MotoGP™ (@MotoGP) September 26, 2018
"I tried the first edition of the 2019 engine, yes, but I think and I hope it's not the final one, because it's very similar [to the 2018 one]," he said, as quoted by Autosport.
"I hope they continue to work, because the engine for me is a problem for us. They have to make it better.
"Unfortunately in testing we haven't tried many things for next year."