Substitute for celery in soup
Honestly, I'd say there is no substitute that you're likely to have on hand. The unique qualities of celery are mostly the cellulose crunch it provides, and the suggested substitutions like daikon, radish, bok choy or the like are even less likely to be in your pantry. 

Most likely you'll just have to go without or make a trip.

If it is just the taste in that recipe you're going for, you could use celery seed if you happen to have that on hand.  

Reference:  http://www.thekitchn.com/my-celery-dilemma-154426