Kubica has had a number of operations since an accident in a rally car in February 2011 and Alonso believes that it's difficult to set a date for the Pole's return.
The Spaniard told F1 Racing Magazine: "It's very hard to say whether Robert will return to 100% fitness and be able to race again in Formula 1.
"Now and again we speak and I know how much it hurts him to be so far away from what has always been his world.
"He has to keep calm and think first and foremost about recovering total functionality of his body, then he can think again about racing."
Kubica has driven for BMW and Renault in Formula 1.