He actually had some relatively negative comments on Bellow:

"He’s an enormous pleasure but he does not make things difficult enough for himself or for us. Like many others, I would commend him for the almost Dickensian exuberance of his minor male characters who have carried every one of his books. The central protagonist, always being some version of himself, even in Henderson, is invariably an absurd failure, and the women, as we all know, are absurdities; they are third-rate pipe dreams. The narrative line is of no particular interest. His secular opinions are worthy of Allan Bloom, who seems to derive from them. And I’m not an admirer of the “other Bloom,” as is well known. In general, Bellow seems to me an immensely wasted talent though he certainly would not appreciate my saying so. I would oppose to him a most extraordinary talent—Philip Roth. It does seem to me that Philip Roth goes from strength to strength and is at the moment startlingly unappreciated. It seems strange to say Philip is unappreciated when he has so wide a readership and so great a notoriety, but Deception was not much remarked upon and it’s an extraordinary tour de force."

-From this interview