Salford issued a statement on Tuesday afternoon, the day after League Express newspaper reported Evalds was on his way to Castleford.
"In respect of this player movement, Salford Red Devils are extremely disappointed with the decision made by Niall Evalds and his representative for the full-back to end his time at the club following the conclusion of the 2020 season," the statement said.
"Following a period of negotiations during February and March, Evalds was 48 hours away from signing a new deal to stay at Salford for a further three years, which had been agreed by all parties.
"A late request was then made for Evalds and his representative to have additional time to consider his options, which the club honoured.
"In April, further discussion took place and an offer was made. Both offers significantly increased the value on his current contract at the club, in addition to incorporating a testimonial and other benefits, which recognised his prior contributions.
"Salford Red Devils were led to believe that the deal would be successfully concluded on this basis. It was therefore extremely disappointing, after months of negotiations, when it was revealed that the player had then signed for a competitor on Saturday morning."
It added: "It is undoubted that Evalds’ progression under head coach Ian Watson and incorporation within a system which allows him to thrive, has assisted him, along with other players, in gaining recognition across the game.
"Salford Red Devils thank Evalds for his service and wish him all the best in the remainder of his career."
Salford were clearly upset at the news being leaked.
Their statement insisted: "Confirmation of player departure details would not ordinarily take place at this stage in the season. In this instance however, the player involved is one with a significant length of tenure at the club and the decision has therefore been made to disclose our actions on this matter, for the benefit of our loyal supporters and sponsors.
"Salford Red Devils’ stance remains firmly against the leaking of player movements, with early announcements themselves causing disruption across the competition. The club will therefore continue its policy of withholding external recruitment announcements at this stage in the season, except where beneficial to the players and organisations involved."
Evalds will link up with former Salford teammate Derrell Olpherts at Castleford.
Olpherts has scored six tries for Tigers this year, one more than Evalds' Salford tally.
Evalds was joint-second top try scorer in Betfred Super League last year, with 22 and played in Salford's Grand Final defeat by St Helens.