The Red Devils midfielder is hoping to beat the Czech Republic to a spot in the playoffs, but a formidable task awaits them if they are to grab second place in Group I
Craig Levein's men are a point ahead of the Czech Republic with one fixture remaining, but face a sizeable task given that the reigning world and European champions have an unblemished record in Group I.
"To speak about getting a point, although it might be a great result, might be irrelevant; it might not be enough," Fletcher told reporters.
"We know going into the Spain game that it is in our hands and if we win we will finish in second place.
"But we might lose and be in second place and we might draw and not be in second place so it's going to be one of those nights.
"We can only try to do our best and we will give it everything and although people are maybe writing us off, while there is still a chance we will keep believing."
Fletcher picked up an ankle injury during the 1-0 victory over Liechtenstein on Saturday, but is confident of being fit to face Spain and reiterated the difficulty of the task facing the team on Tuesday.
"We are under no illusions," he said. "There is no question that Spain are the best side we've played against, they are European and world champions and they have not been beaten in a long time.
"They will have the majority of possession. I watched them against the Czech Republic the other day and they had 65-70% of the ball. So we have to go with a game-plan.
"If we open up too much and try to take the game to Spain then I think we would get punished and be out of the game fairly quickly. So I think in that respect it is going to be difficult.
"The biggest thing is that we won the match against Liechtenstein the other night which put us into second place, and it puts the pressure on the Czech Republic.
"Hopefully Lithuania do us a favour as well which would be nice."