Below is your initial response:
I fear we're going down the path of messy semantic arguments here, but here we go...
1) You both agree that Santana will likely "struggle mightily." But then you said "probably not"?
2) Your subsequent comments suggest that he might never be the same pitcher.
3) Lynn's TJ surgery and aftermath is far more problematic than Santana's knuckle joint lock and pain.
4) Nevertheless, given that Santana is 35 and half years old and a 2019 Free Agent, one would logically follow your analysis and conclusions that Santana might forever "struggle mightily", possibly forced to sign minor league make-good deals going forward. I'd call that outcome career-threatening.
5) Certainly Lynn's FA status and 2018 performance stats which are at or near the bottom of all SPs in many categories are career-threatening, until he can prove otherwise. Not many opportunities for a command-free SP.
6) Perhaps we can all agree that Santana might struggle initially upon his return, the injury and long layoff causing some level of struggles for a month or more.
7) However, at this point, I prefer the very positive and independent medical opinion for likely eventual full recovery for Santana, with likely results in 2018 a shade or two below his career averages (ala Steamer/Depth Charts).
1) Nothing contradictory there. My opinion can differ from the most likely outcome. In this case it does.
2) I don't think he'll ever be the same pitcher, no. That won't stop teams from giving him mlb opportunities to fix himself though.
3) One may be more problematic than the other, sure. My point is not all pitchers are the same after major surgical procedures.
4) Fair enough. But I was going with career threatening in a medical sense. If we're broadening it this far, then yeah, it could limit his opportunities to further his career.
7) That's fine. You should prefer whichever opinions you want. That has nothing to do with my opinion, or that of the poster you initially responded to. This is a message board to share opinions. It's not a clearinghouse for facts. People shouldn't have to somehow be expected to "prove" their opinions here, so long as they are not trying to pass them off as facts.