The teenager has been arrested in connection with an incident just off Fitzalan Square, where a 28-year-old man was found with life threatening injuries on Monday afternoon.
The 18-year-old was arrested on suspicion of assault and has now been bailed pending further enquiries.
The 28-year-old man remains in hospital in a critical condition.
He was discovered unconscious and with a serious head injury at the bottom of the stairs that lead off Fitzalan Square and down towards Pond Street, at around 1.20pm on Monday, 6 June.
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Detectives are still keen to speak to anyone who may have been in the area around the same time.
Anyone with information is asked to call 101, or Crimestoppers anonymously on 0800 555111, quoting incident number 128 of June 7, 2016.