Pro Football Focus has their grades up for Eagles Vs Seahawks and for the most part what they found regarding the Eagles tackling was pretty much what we could have expected. Their most scary finding however, is that there isn't one really bad offender. They're all bad.
If it's good enough for Vince Lombardi to say it's good enough for me; football comes down to blocking and tackling, and tackling is not something the Eagles are very good at. Here they missed another 12 tackles, further reinforcing the fact that if it wasn't for the Tampa Bay Buccaneers, they would be the worst tackling team in the league. They managed to spread the missed tackles around, with only Jamar Chaney (-0.1) missing more than one as a sign of just how deep the problem lies. You take away those missed tackles and life becomes a lot harder for the Seahawks.
So 12 missed tackles. Now, missed tackles are a somewhat subjective stat, but PFF's estimation would mean the Eagles missed one fewer tackle against Seattle than they did against Buffalo, in which Sheil Kapadia counted 13 missed tackles. So it was only the second worst tackling performance of the season for the Birds....
But I think the point to take away from this is that it's a systemic problem.