The coach, who led Portugal to their Euro 2016 triumph in France two years ago, also confirmed that captain Cristiano Ronaldo will start Thursday's friendly at home to Algeria, their final warmup match.
Ronaldo, Portugal's most capped player and record scorer, missed the first week of training and the friendlies against Tunisia and Belgium after playing for Real Madrid in the Champions League final against Liverpool at the end of last month.
"The same thing happened before Euro 2016 with Ronaldo and Pepe, when they both played in the Champions League final," Santos told reporters. "Ronaldo will definitely start the game."
Ten players from the Euro 2016 squad have missed out on the trip to Russia, either due to injuries or a loss of form at club level, and Santos has already said it is proving more difficult to pick the team than it was two years ago.
"I have a lot of doubts, I have 23 players and full confidence in each of them," he said "There is one week to go, we will have to see what happens in training.
"Of course, there is a basis for the team and some players are closer to the starting lineup. But it would not make any sense at this stage to predict the 11 as there are so many uncertainties."
Portugal face Spain in Sochi on June 15 in a group which also includes Morocco and Iran.
(Reporting by Brian Homewood; Editing by Toby Davis)
