Visable indication of Battery Condition

While reassembling my iPhone 5 after replacing the power / volume controls cable assembly, I noticed that the battery was "puffy". The plastic cover was inflated and would depress if pushed. Is this an indication that I damaged the battery during removal (cells off-gassing). The battery sat out of the phone for about 12 hours. (disassembled in evening and began reassembly in morning).

I do not want to completely reassemble the phone, just to have the battery fail or burn up on re-power/charging. If I had a replacement battery I would probably swap it.

Anyone with experience on this. My first replacement attempt, so I have nothing to draw upon.