But, by the time that he had to decide on a name for his second son Efraim, some kind of metamorphosis had taken place within him. The distinction is most telling. While there is little doubt that Yoseph stayed throughout all his life first and foremost a Jew, the anti-Jewish surroundings of Egypt obviously had some influence on him. He had to adapt himself towards his new environment and obviously this may have endangered his own identity. Often, a person himself is unaware of very slight changes.
Assimilation is a slow and, at the start, an unrecognizable process.
It is only when others make us aware of it, that we start to realize what has happened. While the other grandchildren were raised in his own home and in the land of Israel, Menashe and Efraim were born in a foreign country and never had seen their grandfather and never experienced his thoroughly Jewish home. Surely this must have worried Yacov greatly. The question how these grandchildren would stay Jewish in such surroundings must have been constantly on his mind. This, however, does not explain why he seems to favor Efraim over Menashe.