A former user of the day ward at the Peter Hodgkinson Centre, which she said "saved her life", said she is concerned the recovery college will not meet mental health needs in the area.
The trust said the number of users has fallen sharply but it will continue to take referrals until the college opens.
NHS-run recovery colleges are aimed at educating mental health sufferers.
The day ward at the Peter Hodgkinson Centre, based at Lincoln County Hospital, deals with patients who are on their way to recovery and trying to get back into the community.
The general manager of adult mental health services for the Lincolnshire Partnership NHS Foundation Trust, Louise Bussell, said the service is not closed "in any way" but there were only two referrals last month.
"To run a service for a couple of people is quite difficult when we want to build up day services to meet a bigger need.
"It isn't running as it was but the staff that were in it are now working in other services of the hospital."
She said people were instead choosing to use a network of more local services which have opened recently across Lincolnshire.
She added: "What we are trying to do is create something that would be even better in the future."
Former patient Ruth Swift said the recovery college would do a "good job" but would not be as helpful as the ward because it is "based on educational rather than medical".
"When I was attending it was a lifesaver because it was somewhere where you could go five days a week and be safe," she said.
Courses start in July but the centre will continue to take referrals until the college becomes fully operational in September.
A college for people with mental health problems is to replace a day centre in Lincoln, the NHS trust has said.