It may this line from E. Franklin Frazier, 1962:
The Negro, according to Adams, could only be partially assimilated or, in our language, integrated but not assimilated.
To which his definition of terms seems important:
How does integration differ from assimilation? Assimilation involves, of course, integration for it is difficult to see how any people or group can become assimilated without being integrated into the economic and social organization of a country.
— E. Franklin Frazier: "The Failure of the Negro Intellectual", (1962)
While this might look like a second hand quote, Adams being the original, compared to the line in question above, the Adams quote for comparison looks much less like a match:
Thus Charles Francis Adams, referring to the race problem in an address at Richmond, Va., in November, 1908, said:
[…] That theory is now plainly broken down. We are confronted by the obvious fact, as undeniable as it is hard, that the African will only partially assimilate and that he cannot be absorbed. He remains an alien element in the body politic. A foreign substance, he can neither be assimilated nor thrown out.
— Robert E. Park: "Racial Assimilation in Secondary Groups With Particular Reference to the Negro", American Journal of Sociology, Vol. 19, No. 5, 1914), pp. 606–623. jstor