Rotherham United on high alert over lack of numbers as season draws closer
The Millers only returned to training last Thursday but will already be without Curtis Tilt and Angus MacDonald for the start of their League One campaign.
Tilt, who was returning to the club after an impressive spell with Wigan last season, suffered a bad hamstring injury while on international duty with Jamaica.
MacDonald has also picked up an unspecified problem which will keep him out of the league opener with Plymouth on August 8.
Warne said: “Curtis severely pulled his hamstring while he was away.
"That's a 12-week injury so we won't see him before October.
"Angus isn't fit either. There's a little problem with him. He's going to miss the start of the season also.
"We're down to two centre-halves so that is something we're going to have to address. We need defensive cover."
With Clark Robertson leaving the club at the end of his contract last season, and subsequently joining Portsmouth, the Millers have only Richard Wood and Michael Ihiekwe on their books.
Warne said he needs at least eight new signings to rebuild his squad after 10 players left at the end of the club's Championship relegation.