I think it is important when evaluating a quarterback to take two views. The first is the here and now. The second is the longer term perspective. There is very little one can do to spin Mark Sanchez's play positively. He is play extremely poorly and is hurting the team. How is his long term development shaping up, though? Looking at some basic statistics, Mark's early career looks eerily similar to Eli Manning's.
Sanchez: 53.8 completion percentage, 0.6 TD/INT ratio, 63.0 rating
Manning: 48.2 completion percentage 0.66 TD/INT ration, 55.4 rating
Sanchez: 53.3 completion percentage, 1.33 TD/INT ratio, 74.2 rating
Manning: 52.8 completion percentage, 1.41 TD/INT ratio, 75.9 rating
None of this guarantees anything, but it's not all doom and gloom from the long term view.