Ok, Could work but I would suggest that you peel the celery sticks first to get rid of the ' stringy' bits and cook well.Substitute weight for weight. I believe CWS is correct in that it is Celeriac asked for in the recipe. Try to get some and make this vegetable your friend. It looks kinda scary and is sometimes known as ' Elephants Foot' but once peeled it is so versatile and delicious. I use it a lot as a substitute for potato when I need to watch the waistline for a while, as you do, LOL It would make a creamier, richer soup than Celery but I adore soup so would eat either. Good Luck.
Life is like good wine.......best taken with friends x