Rangers captain Steven Davis made an appeal to free agents as his side look for squad strength after crashing out of the Scottish Cup.
Dundee United travelled to Ibrox on Sunday and were 2-0 victors as the 33-time champions slumped out of the tournament.
Top-scorer Nikica Jelavic left for Everton during the January transfer window leaving Rangers with a hole to fill in attack but manager Ally McCoist was unable to secure a new recruit before the deadline passed.
And now Davis, who is eager for the squad to be bolstered, has acknowledged the club's only chances of adding any depth will come through free agents.
"The window is closed and it's only frees that are going to be available but it would be nice to see a bit of investment in the team and try to add to what we've already got," he said.
"The problem is that we've got a lot of key players out as well and that's something that's gone against us this season."
The SPL champions are in the middle of an ongoing tax tribunal which could leave a £49million bill hovering over the club and midfielder Davis admits Rangers' situation can be frustrating.
"That's something that has been hanging over us for a long period of time now," he added.
"But it's disappointing to see players like Jelavic, a very important player, leave without really getting anybody in. That was a bit of a blow for the players."