Humphreys, 35, who made a club record 542 appearances for Pools, will end his 12-year spell at Victoria Park following the club's decision not to take up an extra year's option on his contract.
"The news surprises me," Cooper said. "He's a true Hartlepool legend is Ritchie. He did ever so well for the club over the years.
"You cannot get a more consumate professional. But he was also a very good coach and I know that was something he wanted to get into."
Former Sheffield Wednesday striker Humphreys joined Pools in 2001 and played a key role under Cooper during the Scot's first spell in charge of the club, who were beaten by the Owls in the 2005 League One play-off final.
"He was an excellent player for me," Cooper added. "He could play in numerous positions, wide left midfield, central midfield, left-back, striker and he never let you down.
"He always gave his lot and is a lovely guy as well. He was a real fans' favourite. I think they named a street after him in Hartlepool and you can't get much better than that."