Craig Mackail-Smith grabbed all the headlines with his superb headed winner against Liechtenstein which has taken the Scots' hopes of making the European Championship play-offs to a nerve-shredding climax in Alicante.
Rangers star Naisy turned in an unusually profligate performance to spurn three glorious chances in the Group I crunch in Vaduz.
But Stein, the last man to score a hat-trick for Scotland in a World Cup qualifier in Cyprus back in 1969, has backed the Light Blues ace to slip in under the Spanish defensive radar and shoot the Scots to Euro glory.
He said: "There is no point in playing down how tough this is going to be. But we have to focus on what we can do and Steven Naismith could be the key player in Alicante tomorrow night.
"Sure, Naismith passed up on a few he would have been disappointed to have missed, but the point is he was in there under the radar all night long.
"It is vital when you play one-up that the supporting player has an element of elusiveness about him and Naismith certainly has that.
"The chances he missed will have made him even more determined to get on the scoresheet against the Spanish.
"What I liked about Naismith on Saturday night was the way he kept showing where it hurt even although he didn't get the breaks he deserved. That takes a lot of bottle and no little belief.
"I feel if Scotland are going to get anything tomorrow – and we will need to score – then Naismith is going to be vital."
The Scots have only ever tasted success on the plains of Spain once, and that came way back in 1963 when Denis Law inspired us to a 6-2 victory in a friendly.
Stein, in his pomp a striker who put personal safety a distant second to his hunger for goals, believes that new Dark Blues hero Mackail-Smith has a bright future on the international stage.
The Rangers Cup-Winners' Cup hero said: "I liked Mackail-Smith a lot on the evidence of the Liechtenstein game. For a start, he worked his backside off and did a lot of the dirty stuff really well.
"Craig's attitude was bang on. When it came to the goal he got up really well and showed great composure with the header. In fact, I would have been happy with that one myself.
"Obviously Kenny Miller is all but out and the fact that Craig has got a goal on his first start is very timely.
"I know he has a back niggle that is causing hamstring discomfort but from what I hear he will play.
"When you go into games as big as this it is vital you have players playing at the top of their game and full of confidence. Mackail-Smith is in that place right now and that is a bonus for Scotland."
Stein, who never played at a major tournament with Scotland, hopes Levein's men can make the most of the oppor-tunity within their grasp.
"Obviously I never played in a major finals tournament and that is something that still hurts," the 64-year-old said.
"What I would say to the boys is that this is a great chance. The Spanish are over the finish line and their focus may not be where it should be.
"We rattled the Spanish badly at Hampden even though we lost, so we have it in our locker. Belief will be everything."