The Light Blues were looking to bounce back from Sunday's defeat at Hearts, but with Alfredo Morelos on the bench, they again laboured through the first 41 minutes.
That changed when Defoe buried an expert finish and he followed that up two minutes into the second half as he set up Scott Arfield for the second to put the game to bed.
However a worried hush fell over Ibrox on the hour mark when he slumped to the turf with a calf injury.
The veteran strike star was stretchered from the field, which Gerrard admitted made the situation look worse, but it is still a concern nonetheless.
Gerrard is hopeful the injury will not keep the veteran out for too long but admits he may need to go into the transfer market before Friday's deadline in search of extra cover.
Gerrard - whose side remain five points behind Celtic with a game in hand - said: "I'm concerned but I don't know how long the injury will keep him out.
"I don't think it's as bad as going off on a stretcher makes it look. He's walking about in the dressing room.
"He'll be scanned in the next 24 hours and we'll know more about it then. But he has got a calf injury - whether it will be a week, two weeks, three or four weeks, I don't know.
"I haven't spoken to our sporting director Ross Wilson about the transfer situation yet. We have a couple of things brewing besides that."
Meanwhile, Morelos started on the bench as the Light Blues took no chances with a nagging calf complaint but he came on late on and was only denied a goal by Staggies keeper Nathan Baxter.