  In this letter, we reconstruct the formation pathway of MRG-S0851, a massive,
$\log M_*/M_\odot=11.02\pm0.04$, strongly lensed, red, galaxy at
$z=1.883\pm0.001$. While the global photometry and spatially-resolved outskirts
of MRG-S0851 imply an early-formation scenario with a slowly decreasing or
constant star-formation history, a joint fit of 2D grism spectroscopy and
photometry reveals a more complex scenario: MRG-S0851 is likely to be
experiencing a centrally-concentrated rejuvenation in the inner $\sim$1 kpc in
the last $\sim$100 Myr of evolution. We estimate $0.5\pm0.1\%$ of the total
stellar mass is formed in this phase. Rejuvenation episodes are suggested to be
infrequent for massive galaxies at $z\sim2$, but as our analyses indicate, more
examples of complex star-formation histories may yet be hidden within existing
data. By adding a FUV color criterion to the standard U-V/V-J diagnostic,
thereby heightening our sensitivity to recent star formation, we show that we
can select populations of galaxies with similar spectral energy distributions
to that of MRG-S0851, but note that deep follow-up spectroscopic observations
and/or spatially resolved analyses are necessary to robustly confirm the
rejuvenation of these candidates. Using our criteria with MRG-S0851 as a
prototype, we estimate that $\sim$1\% of massive quiescent galaxies at $1<z<2$
are potentially rejuvenating.
