I have no idea what the real story was, but Jefferson wound up on Gregg's bad side and was put in the doghouse permanently. JJ bitched about it towards the end of his time here, and Gregg said "tough sh*t", paraphrased, of course.
FWIW, Gregg's hiring may have a bigger mistake than the decision to fire Starr. Part of the reason he was brought in was his reputation of being tough and hard-nosed, and supposedly that's just what the Packers needed. He was that indeed, but it translated poorly to the players, and we pretty much sucked the next few years until the Majik era.
Without knowing the details, my recollection is that Jefferson did not have near the stats he had while with the Chargers and I was disappointed. I thought I heard he wasn't much of a practice player and it caught up to him.
Man, I thought the combination of Dickey throwing to Lofton, Jefferson, and Coffman was going to be unstoppable. I recall it just did not turn out that way.