I am writing a paper using the springer journal LaTeX template. For some references I want to use in-text citation like this:

but for some reason, when I try this in my document I get the following:

I can't seem to figure out where the problem lies. I tried using \citeauthor and \citeyear, but that does not solve the problem unfortunately.
Below is a MWE:
\documentclass[]{article}
\usepackage[numbers, sort, comma, square]{natbib}
\begin{document}
bla bla bla \citet{aaronson1998}.
\bibliographystyle{unsrt}
\bibliography{bibfile}
\end{document}
BiBTeX entry:
@article{aaronson1998,
title={Translation, validation, and norming of the {D}utch language version of the
{SF}-36 Health Survey in community and chronic disease populations},
author={Aaronson, Neil K and Muller, Martin and Cohen, Peter DA and Essink-Bot,
Marie-Louise and Fekkes, Minne and Sanderman, Robbert and Sprangers, Mirjam AG and
te Velde, Adrienne and Verrips, Erik},
journal={Journal of Clinical Epidemiology},
volume={51},
pages={1055--1068},
year={1998},
publisher={Oxford; New York: Pergamon Press, c1988-}
}
