Following the 1-1 draw at Stamford Bridge last month, the tie is evenly poised going into the second-leg and Barca head coach Ernesto Valverde received a major boost with the return of his two key players.
The Iniesta-Messi combination forced the first-leg equaliser in London but there were doubts over their inclusion for the fixture, with Iniesta struggling with a hamstring injury and Messi being left out of their weekend victory over Malaga - due to the birth of his third son.
Both players have since been recalled as revealed on their official Twitter account but Valverde admitted the inclusion of the Spain international is a huge risk.
"Tomorrow is a must-win situation so you need to take risks, as long as they are not too excessive," he told reporters, as quoted by Goal. "But you also do not want to compromise the safety of the player."