After injuring his back twice in the last three years, his team signed the most overpaid player in the league to fill his spot (RG Leonard Davis), and the emergence of RT Marc Columbo after the ex-Bear had spent many years trying to come back from injuries himself, the Dallas Cowboys found they had no more use for ex-Packer, three time Pro Bowl RG Marco Rivera.
Rivera's departure was the beginning of the end for a team that had just won three consecutive NFC North titles. Due to his age and the high mileage of playing in over 100 consecutive games, it seemed like Rivera would attract little interest in free agency. Plus Rivera had taken a step back in 2004 as he was pushed around more that season than in the previous one. However Detroit and Dallas got into a bidding war for him and Dallas won by offering a $8 million signing bonus.
It was a bitter pill to swallow by letting him leave and only signing the fumes of RG Matt O'Dwyer's career and drafting 7th round G William Whitticker to replace him. Both players were failures, but it did lead to the drafting of promising RG Jason Spitz. Letting Rivera leave was the right thing to do because the Packers had salary cap problems anyway and he was being rewarded for past performance when his NFL career seemed unlikely to last too much longer.
Although he started 30 of 32 games in Dallas over the last two seasons, his back was no longer able to hold up. Since he is unable to pass a physical at this point, it seems that his career is over. Rivera doesn't deserve to end his career like this, but that's how it goes sometimes and it sounds like he is taking his release with class. Hopefully he is just as successful with his 2nd career as he was with his first one.