The National Institute of Health COVID-19 treatment guidelines state that there have been an increasing number of reports of patients who experience persistent symptoms and/or organ dysfunction after acute COVID-19. Data about the incidence, natural history, and etiology of these symptoms are emerging, and more research is needed to better understand the pathophysiology and clinical course of post-acute COVID-19 sequelae in order to identify management strategies. 
Persistent symptoms seen in COVID-19 patients after treatment include fatigue, dyspnea, and neuropsychological symptoms, and are seen in 35% of outpatient patients and 87% of hospitalized patients. While these symptoms are commonly reported, there is no certain way to diagnose the underlying reason as changes in metabolism, hormonal balance, and inflammatory messengers directed against one's own body and changes in brain function could contribute to the suffering of patients, especially those with fatigue.
Currently, it is too early to diagnose post-COVID syndrome. Symptoms must last at least six months, and those cases have not yet been reported. While a "post-COVID syndrome" cannot be diagnosed, it is possible that critically ill COVID-19 patients will have long term physical, cognitive, and psychological consequences once treatment is complete.
In conclusion, there is sufficient evidence for the possible existence of a post-COVID syndrome.