Centre-back Stefanovic damaged his cruciate ligaments last November, Hoolahan sustained ankle ligament damage in March while Drury has suffered with a persistent knee problem since October 2007.
Spencer said: "We're hoping Dejan will make a start to the pre-season and hopefully his fitness will continue progressing as well as it has.
"Adam's done very well, he had an operation at the start of May which tidied things up and he's been running at three-quarter pace.
"Wes is around 10 weeks post operation and is looking to step back into full training. There have been no reported problems with him so we are hopeful he'll continue and be fit for pre-season."