Silva is still recovering from an ankle injury he sustained while scoring his side’s second goal in their 4-0 Group A win over New Zealand.
The Manchester City midfielder trained alone at Kazan’s Tsentralny Stadium on Tuesday in a bid to accelerate his recovery but coach Fernando Santos is unwilling to take any undue risks.
“We still have 24 hours. He’s reacted well to his recovery but we have to wait,” Santos told a news conference.
“Time is very important in this case and we’ll decide what’s best tomorrow [Wednesday].
“I’m just thinking about the game, full stop. The most important thing for me and my team is to win.”
Santos also confirmed that full-back Raphael Guerreiro has not recovered from the ankle blow he sustained in the opening draw with Mexico, meaning he will also sit out the match.
Portugal’s defence has been further damaged by the suspension to Pepe, with the experienced centre-back serving a one-game ban for collecting two yellow cards in the group stage.