Sunderland boss Alex Neil has said striker Jermain Defoe gave no indication he was planning to retire.
The 39-year-old former England international returned to the Black Cats on 1 February but hung up his boots last week.
He failed to score in his seven appearances for the League One side.
"I'll be honest, it was a bit of a shock. There wasn't a lot of conversations prior to it," Neil told BBC Radio Newcastle.
"He trained that day and he trained well and then the next minute we got word that he felt that he wanted to call it a day.