Key defender Shittu has suffered cruciate knee ligament damage.
And Cureton was on his way to Rangers before negotiations with his Korean club, Busan Icons, hit the rocks.
A supporter agreed to pay Cureton's wages but the former Reading man's move to Loftus Road was called off after a dispute over his transfer fee.
Shittu's partnership with Clarke Carlisle has been vital to Rangers' push for promotion from Division Two and his injury is a huge setback for the club.
"It's a bitter blow," said Rangers manager Ian Holloway.
"He's one of our most important players but we have to try and get on with it without him now."
Holloway will now give Terrell Forbes, who has operated mainly at right-back, an extended run in his preferred central defensive role.
And the R's boss is considering his next move after seeing his long-running bid to sign Cureton, who played under him at Bristol Rovers, come to nothing.
"We thought we were going to do that deal but it's off," Holloway admitted.
"It doesn't look like we're going to be able to do that now.
"You won't find me complaining about these things though. We've still got good players and we need to focus on getting to where we want to be."