Just thinking out loud, I would think for anyone betting serious money, whether Gronkowski plays or not is a pretty significant development. Why would you wager significant money 11 days out with that big piece of uncertainty?
I have come up with 2 possibilities.
Senerio #1. If you were convinced that the patriots were the superior team, with or without Gronkowski and were going to win easily, either way, you would want to place your wager while there is uncertainty. Laying 5 or 5.5 is significantly better than laying 7 or 7.5, if Gronkowski was cleared.
But that is clearly not what this bettor did.
Scenerio #2. If you were convinced the Eagles cover or win with or without Gronkowski, you would want the +5.5, rather than 3 or 3.5 should Gronkowski be ruled out.
This may be what this bettor had in mind. On the other hand maybe he is just some rich Eastern European, "kid (they said under 30) with too much money and didn't put much thought into it.