I looked up the 1998 thesis, in chapter 3.7 he writes about "Complications and post-viral illness" (hastily translated by me):
His sources:
Collier, R (1974): The Plague of the Spanish Lady. The influenza Pandemic of 1918-1919. Macmillan.
Det civile medisinalvesen (1922): Sundhetstilstanden of medisinalforholdene 1918, NOS VIL.58.
Galowsky, E (1919): Haaravfald etter influenza. Tidsskrift for Den Norske Lægeforening. 39(13): 531-532.
MAttock, C., Marmot, M. and G. Stern (1988): Could Parkinson's disease follow intra-uterine influenza?: a speculative hypothesis. Journal of Neurology, Neurosurgery and Psychiatry. 51.735-736.
Ravenholt, R.T. and W.H. Foege (1982): Before our time. 1918 influenza, Encephalities Lethargica, Parkinsonism. The Lancet, 16 Oct., pp. 860-864.
At least in this chapter he does not include a source for the statement that people were not able to do tasks of daily living or work for months (years is not mentioned). It may be it's written somewhere else, but this chapter seemed the most likely based on the title

Unfortunately the document seemed to be scanned and I am unable to search through it. Potential long-term problems following the illness is not important enough to be part of the conclusion and what future research should focus on.