Or HaChaim on Genesis 48:18/אור החיים על בראשית מ״ח:י״ח

1 ויאמר יוסף..לא כן אבי, Joseph said to his father: "not so my father, etc." We need to understand why Joseph did not simply say: "this one is the firstborn," instead of the preamble "not so my father."

2Joseph entertained two possible doubts. 1) Inasmuch as Joseph and his sons were facing Jacob, and it was customary for Joseph to have his firstborn on his right and the younger one on his left, they would appear in the reverse position when viewed from Jacob's vantage point. Jacob may therefore have reasoned that in order to place his right hand on the older one of the two he had to cross his hands. 2) On the other hand, Joseph reasoned that it was possible that his father crossed his hands in order to place his right hand on the head of Ephrayim, deliberately, thus giving him preference over Menashe. In view of this Joseph first told his father that if he had assumed that Joseph had placed the boys as he usually did, i.e. Menashe on his right, לא כן אבי, i.e. he did not do so this time. If, on the other hand, Jacob had deliberately placed his right hand on the head of Ephrayim, Joseph registered his protest by saying: "place your right hand on him for he is the firstborn." He wanted to know why his father denied his firstborn what he thought he was entitled to.