I don’t know who you are using on the SJU-EIS leg. What I recommend is the night before use flight aware to check the status of the leg. Check it again in the morning as you board your US flight. If your proactive you may be able to audible before leaving the US. If issues come up later have the phone numbers of a couple of charter airline options. They can save the day!
Looking at tomorrow as a example IC has already cancelled their morning flight. If you catch this in advance you might get rebooked on that leg or could ask to fly direct STT.