Legions of the Polish driver's fans are disappointed that Williams elected to instead sign the heavily-sponsored Russian Sergey Sirotkin for 2018.
However, Kubica's co-manager, 2016 world champion Nico Rosberg, told Auto Motor und Sport that the test and reserve role the 32-year-old has secured is still positive.
"Robert will be able to do many kilometres.
"For him, it's a mega contract and definitely a step in the right direction."
Kubica had put together an $8m (£5.8m) sponsorship purse for the race seat, but the publication claims that he will actually be paid by Williams this year.