That is very hard to say based on the info you've provided and the situation. Can you list water params for ammonia, nitrite, nitrate, and calcium to go with the pH? There will be many things to determine the long term health of your puffer, including the variety in food. That fish can't live on just krill and remain healthy. Puffer's need a variety in their diet, things like Formula 1 frozen food, raw frozen squid, raw frozen scallops, raw frozen shrimp... krill alone isn't enough to sustain it.
The combination of water quality, nutrition, and stress levels are all going to play a part in how much damage is done, has been done, and any recovery the fish may make from it.
How long was the pH low? How long was the fish having problems? Do you know why the pH dropped? What was the pH before it dropped to 7.4? All of these questions need to be answered before anyone can offer even a guess. Anything you can think to tell us about the tank, such as what substrate you have, how much live rock is in the tank, water temp, etc will all help us to help you much quicker.