Ms Reissmann is claiming that she was unfairly sacked by Manchester Mental Health and Social Care Trust in June last year for speaking out about cuts and the privatisation of health services.
She was found guilty of gross professional misconduct following a six day internal disciplinary hearing, which was chaired by the trust's then chief executive Sheila Foley.
NT understands Ms Foley, who resigned from the trust earlier this year will also give evidence at the tribunal.
The employment tribunal will start in Manchester on Monday and is expected to last for a week.
Unison Manchester community and mental health branch secretary Caroline Bedale said: 'Karen's case raises issues about how the NHS is no longer a national cooperative body…NHS trusts are not buying and selling cornflakes.
'We are providing a public service. We should not be silenced by commercial interests. It is not good for staff, not good for patients and not good for the NHS'