  Understanding and controlling the electronic properties of two-dimensional
materials is crucial for their potential applications in nano- and
optoelectronics. Monolayer transition metal dichalcogenides such as WS$_2$ have
garnered significant interest due to their strong light-matter interaction and
extreme sensitivity of the band structure to the presence of photogenerated
electron-hole pairs. In this study, we investigate the transient electronic
structure of monolayer WS$_2$ on a graphene substrate after resonant excitation
of the A-exciton using time- and angle-resolved photoemission spectroscopy. We
observe a pronounced band structure renormalization including a substantial
reduction of the transient band gap that is in good quantitative agreement with
our {\it ab initio} theory that reveals the importance of both intrinsic WS$_2$
and extrinsic substrate contributions to the transient band structure of
monolayer WS$_2$. Our findings not only deepen the fundamental understanding of
band structure dynamics in two-dimensional materials but also offer valuable
insights for the development of novel electronic and optoelectronic devices
based on monolayer TMDs and their heterostructures with graphene.
