Loss or draw might make a difference only if Gala fail to win tonight. If they win, then you're absolutely right and a draw vs Real tonight is the same as a defeat. We'll have to beat Gala in Istanbul in both cases.
Beating Real tonight is important for another reason. If we beat them, they will need to play against Gala for a point or 3 points, instead of simply letting the Turks win in Madrid. My fear is that if we don't beat Real tonight, or even worse - if we lose, they really won't have anything to play for in the next round and they might let Galatasaray win. In such case our match in Istanbul will be irrelevant because we'll be eliminated in round 5 already.