Apples and Oranges.
Let's not forget that when Rivera was named coach of the Panthers in 2011, they had just come off of a 2-14 season. The Panthers have shown consistent improvement since then. 6-10 (+4) in 2011, and 7-9 (+1) in 2012, 5 games better over his first two years. Philbin inherited a 6-10 team and took them to 7-9 (+1) in 2012. Rivera's first year improvement was much better than Philbin, and will already beat Philbin in terms of improvement over two years. We may not even finish better than last year. Rivera also plays in a much better NFC South, while we play in one of the weaker divisions.
Bottom line, Philbin inherited a better team and hasn't gotten them to improve that much. Rivera inherited a bad team, and likely has them in playoffs in his third year.