Rovers boss Sam Allardyce would not disclose the names of the players affected but two are believed to be David Dunne and Chris Samba, who both missed Rovers' 5-0 drubbing at Chelsea on Saturday.
"It's definitely swine flu," Allardyce confirmed.
"You've speculated on the names and I'll leave it at that.
"We have had one more case this morning, one more player and there are a couple of staff.
"Other than that, the others that we left behind are recovering well but not available for tomorrow."