The Times says Kay has been with United since 1995 and her pending axing has been described as "brutal" by sources.
Kay was head of team operations, having previously served as former CEO David Gill's executive assistant.
Players and former managers have been hugely supportive and grateful to Kay for her work ethic and energy in supporting the playing and coaching staff.
Kay was said to have been informed of United's decision last week.
Former United captain Ashley Young, now with Everton, has said of Kay: "She's just brilliant.
"She's like the mum to everyone. If there are any problems or there is anything we need sorting out it's a case of 'Jackie can you do this', 'Jackie can you help with this'.
"I'm sure there are times when she looks at her phone when it's ringing and she must think 'no, I'm not answering'. But she's always there for you."