The Brazilian midfielder provided the vital finish in the penultimate minute of the 90, nodding home from a Bruno Fernandes free-kick to seal a 1-0 victory for Erik ten Hag's side.
It had been a night that required plenty of graft and determination until that point, with United resilient in the face of the challenge Forest posed while forging several chances to take the lead earlier in the contest.
Casemiro will undoubtedly grab the headlines for his game-changing contribution, but key to the victory were important and resolute displays from Reds all over the pitch - including those who came into the starting XI following disappointment against Fulham at the weekend.