Chosen Solution
I replaced the battery following Ifixit repair guide and the phone works except for the Power/Sleep button. Phone will boot up when attached to usb charging cable and can be turned off or restarted from General menu. All other functions seem to work correctly except those requiring the use of the power/sleep button. I reassembled the phone twice just in case I had missed a connector with no luck.
You might have noticed the power button cable runs partly under the battery, thus it would be logical to think it might have suffered a microdamage while removing the old battery. Check also volume and vibration buttons as well as the flash light. However a flat cable replacement should fix the problem. You can use the following guide for reference: iPhone 5s Upper Component Cable Replacement
