Former grand prix driver Doornbos knows the back of the grid well after racing for backmarker Minardi a decade ago.
However, he told the Dutch broadcaster Ziggo Sport Totaal that the Manor team should actually make a big step forward in 2016.
"On paper they remain the smallest team, but with the steps they have made, I think they can finally fight the other teams rather than just be at the back all the time," said Doornbos.
"I think they can hope for some points, especially with [Pascal] Wehrlein in the team. I think he's a really, really good driver.
"Haryanto is in the car for the usual reasons: the money he brings and to be a sort of ambassador for Indonesia. We will have to see if he can always qualify within the 107%."
Manor could be set for another financial boost in the days before Melbourne, with reports in Spain suggesting that GP2 driver Sergio Canamasas is in talks about becoming test driver.
According to caranddriverthef1.com, Canamasas is backed by two banks and the Spanish automobile federation RFEdeA.