In this letter, we reconstruct the formation pathway of MRG-S0851, a massive, $log M_*/M_odot=11.02pm0.04$, strongly lensed, red, galaxy at $z=1.883pm0.001$. 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 $0.5pm0.1%$ of the total stellar mass is formed in this phase. Rejuvenation episodes are suggested to be infrequent for massive galaxies at $zsim2$, 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.
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