The Pole - still rated as one of motor-racing's finest protagonists - has been out of action since February 2011 when he suffered a horrendous accident whilst rallying.
Since then, Kubica has undergone countless operations to repair the damage inflicted, with the full extent of his recovery surrounded by uncertainty.
Although recent reports indicated that Kubica has tested in a race simulator, Alonso, a close friend of the former BMW and Renault driver, has admitted that Kubica's comeback still cannot be guaranteed.
"It's very hard to say whether Robert will return to 100 per cent fitness and be able to race again in Formula 1," Fernando told September's edition of F1 Racing Magazine.
"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."