Bayern Munich captain Lahm sustained an ankle injury in the club's 2-0 DFB-Pokal final victory over rivals Borussia Dortmund last weekend, while midfielder Schweinsteiger missed the game because of a knee problem.
Goalkeeper Manuel Neuer also injured his shoulder against Dortmund, and doubts had been raised over the trio's involvement for Joachim Low's Germany side ahead of the showpiece event in Brazil that starts next month.
However, Schweinsteiger believes he will be fit for Germany's World Cup opener against Portugal on June 16, together with Lahm and Neuer.
"We and the doctors are not magicians. So we will have to see how fast the injuries take to heal, but I think everybody is feeling good," Schweinsteiger said at a press conference.
"The most important thing is to feel healthy in the first game. I would not be worried even if Philipp Lahm, Manuel Neuer or I could not play in one of the (pre-tournament) friendlies. I am sure that everyone will be fit for Brazil.
"My injury is getting better, it feels better. But I have to be patient. I cannot go from zero to 100 percent. I have to go step by step. That is the right way.
"Of course it would be better to practice with the team and to do everything but that's the way it is. But we are very experienced and we know what our body needs.
"That's why we are not worried, even if it would be better to be at 100 per cent from the very first day, that's for sure. But I am not worried at all."
Lahm added: "Basically I am feeling fine, except that I am not able to do everything now. That is the case. You need some time and patience, but I have that.
"There is no special schedule. You have to see how the ankle reacts to pressure day by day. That is why you need time. But I am not worried about the first game of the World Cup."
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