Striker Kenny Miller is still struggling with a groin complaint which kept him out of Saturday's 1-0 win over Liechtenstein in Vaduz, a result which means a victory over the world and European champions will guarantee the Scots a play-off place.
Craig Mackail-Smith, who scored his first goal in his first start for Scotland, has a back problem, skipper Darren Fletcher suffered an ankle knock and Barry Bannan had to have a cut on his Achilles glued together.
Should both Miller and Mackail-Smith fail to make it, Blackburn's David Goodwillie will play the lone striker's role.
When asked about injuries, the Scotland boss said: "Where to start? The most doubtful will be Kenny. He hasn't improved from Friday until now.
"I'm not definitely ruling him out but he would be the most doubtful.
"Darren Fletcher and Craig Mackail-Smith will do a little bit (on Monday) and will have fitness tests (on Tuesday).
"I am more hopeful with those two than Kenny but as yet I can't give any definitive answer.
"Barry will do a little bit (on Monday). His problem is whether he can get his boot on so it doesn't open up the injury again, it is in an awkward place.
"We are hopeful that we can put some sort of padding or strapping that will prevent it from opening up.
"It is very difficult when there are so many important players who are doubtful. So it will be Tuesday before I find out."
Fletcher knows Scotland could upset the odds against Spain and still not finish in a play-off spot.
"To speak about getting a point, although it might be a great result, it might be irrelevant, it might not be enough," he said.
"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 is 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, who is confident of shaking off an ankle knock picked up in the 1-0 win over Liechtenstein in Vaduz on Saturday night, reiterated the enormity of the task awaiting the Scots.
"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."