Wales face Finland today and Germany on Wednesday, two matches that could make or break the World Cup campaign.
"Any chances that we have of qualifying will probably evaporate if we do not win this one," he said.
"I have been pleased with the performances in Germany and Russia - but we didn't get anything. We needed a point somewhere.
"Qualifying will be almost impossible if we now do not beat Finland.
"We always realised that this would be a pivotal few days, with two home games. The lads know what is at stake."
Toshack senses the equations narrowing for all would-be qualifiers for 2010.
"This is our first real pressure game, more so than in Germany and Russia earlier in the group where we were not expected to win," he added.
"This one we must win, and we need big performances from Jason Koumas and Simon Davies - our captain (Craig Bellamy) as well.
"They are the experienced ones. We have a lot of young players around, so we if we are going to get the result we want Jason and Simon, in particular, are two we need good performances from.
"I am sure one day Wales will qualify for something. That is the challenge - but don't forget we are in a group with two of the top sides in Europe, so it could have been easier.
"This is a must-win game - there's no doubt about that. We need the experience of those two; they are creative players and they can produce positive things. They are really important now for us.
"They know my feelings. Neither of them has been with us for five months and missed the last two games, where we gave creditable performances against sides seeded higher than us."
Toshack is not afraid to warn his established players there are others pressing for places.
"There are now younger players pushing their way into the side, certainly in midfield, and the experienced ones have pressure on them to perform.," he spelled out.
"Someone like Simon has been around a long time now and he will be frustrated that he has not played in a major finals. But for now they can still do something about it, and I don't think it is asking too much to say to them that now is the time to perform.
"We want them to be winners, to do something. Each year that passes means there is less time to achieve things.
"For the older ones, time is running short. The youngsters, I believe, will get to a finals."
Bellamy and defender James Collins have overcome knee and hamstring injuries to play in a squad which has largely stayed fit since Toshack named it.
Hull's Sam Ricketts is suspended, while West Ham youngster James Collison is out for several weeks with a dislocated knee.
Toshack is likely to play a back four - with Spurs youngsters Chris Gunter and Gareth Bale as full-backs - while Bellamy expects to be the lone striker.
Koumas and Davies will be partnered in midfield by Joe Ledley and David Edwards, and Carl Fletcher could be used in a holding role, with Ramsey on the bench waiting for his chance to shine as a second-half substitute.