For an underdog, one fit attacker can shift the feel of a tie.
Malaysia were always likely to need moments rather than long spells of control. Sumareh suits that kind of game: breaking into space, carrying the ball forward and giving Vietnam something to defend beyond the edge of their own box.
The bigger selection story may still be who is missing.
That leaves Malaysia looking thinner in key areas and increases the chance of fringe players being asked to step up. In a knockout first leg, that can cut both ways: fresh energy helps, but cohesion becomes harder to find against a settled opponent.
Vietnam appear to be slight favourites for good reason.
That does not make the tie one-sided. A first leg at Cheras gives Malaysia a platform, and home support could make the contest more emotional and more awkward for Vietnam than the balance of form might suggest.
The key now is simple: keep the tie alive. If Malaysia can stay compact, get Sumareh involved early and take belief into the second leg, the semi-final may still have a long way to run.