In 2010 and 2011, Finn Raikkonen tried his hand at world rallying, and now former BMW and Renault driver Kubica - still recovering from his 2011 rally crash - is driving for Citroen in the second-tier world rally series.
"Robert's potential to reach the top is very good," Frenchman Loeb told Finland's Turun Sanomat newspaper.
"He is fast, he does need more experience but I think Robert will become faster than Kimi.
"It's still difficult to say, but I can see that he really wants to develop.
"Kimi was really good straight away, but then was not able to improve in the way he wanted. He returned to F1 and won again, which shows just how completely different F1 and rally are.
"Robert is really serious about rally, and is working hard to improve," he added.
Kubica is currently unable to drive a single seater due to the restricted movement of his right arm, but he has been testing at the wheel of Mercedes' F1 simulator.
Mercedes' Toto Wolff played down suggestions the Pole is shaping up for a F1 return with the Brackley team.
"At the moment he's rallying, which I think is his decision at this time," he is quoted by Italy's Tuttosport.
"It's up to him if he wants to stay in rallying or return to Formula One," added Wolff.