According to NST, in his first 14 games (pre-surgery) he had an all-situations GSAA +6.71, allowing 35 GA against 46.93 xGA (3.37 xGA/60), GSAx = +11.93.
In his last 43 games (post-surgery) he had a (NST) GSAA of -6.25. He allowed 127 GA vs. 129.29 xGA (3.35 xGA/60). That's a GSAx of +2.29.
Moneypuck doesn't have a filter to split the season I don't think. They have him at +5.5 GSAx for the whole season. 162 GA vs 167.54 xGA.
I don't think the Jets got particularly worse in front of him later in the season...they weren't good in the beginning, but he was doing his usual thing, saving way above expected. That wasn't happening post-surgery.
Hellebuyck's played well in front of shaky team defense before. He was doing it in the first month and a half this season.