Tub drain knob broke?

Hello, The drain knob on my tub broke when I was taking a shower. I hit it to the up position with my foot by accident and the tub wouldn't drain. I then tried to move it back to the down position to open the drain and the little knob broke right off. The drain assembly then stayed in the down position (gravity). I opened the cover where the assembly is and I can pull it up to drain the tub but as soon as I release it, it goes back down. I rigged the assembly with with a wire to hold it up for now, and as I never use the tub as a tub, only for showering, is there a way to remove the drain plug assembly without removing the whole tub? Thanks, Dan.