Bris Milah in Lech Lecha: Avraham's Transmission of a Spiritual Inheritance to Yitzchak

In Parshat Lech Lecha we learn that Avraham performed the mitzvah of bris mila at age ninety nine (99) following Hashem's command to do so (17:24). In Parperaot L'Torah, R' Menachem Baker asks why Avraham did not fulfill the mitzvah of mila earlier according to the opinion that Avraham voluntarily kept all of the mitzvot in the Torah (see Toldos 26:5; Yoma 28b; though not all commentators agree). R' Baker cites an explanation from R' Yitzchak Zev Soloveitchik (Rav "Velvel" - son of R' Chaim) that Avraham kept all of the mitzvot according to the principle "zerizim makdimim l'mitzvot", but since mila is characterized as a "brit" - a covenant between man and Hashem - and a covenant (by definition) requires two parties - it was impossible for Avraham to observe this mitzvah prior to Hashem's explicit command.

That is, the essence of the mitzvah of "brit" mila, as the very name of the mitzvah conveys, is a "brit" - covenant - between a Jew and Hashem, and this essential "covenantal" aspect would be missing had Avraham independently performed mila without the participation of the other "party" to the covenant - i.e., Hashem (it has been noted that the word "bris" (covenant) appears 13 times in the verses in Bereishit that introduce this mitzvah).

I want to suggest another possible explanation for the delay in Avraham's performance of mitzvat bris milah.