Manager Kenny Shiels was left to rue a lapse in Kilmarnock concentration after losing 2-0 at Ibrox.
The visitors fell behind to a Nikica Jelavic goal from a free kick on 64 minutes and Rangers added a second goal on the break shortly afterwards.
"It's just the set-piece that's done us," Shiels told BBC Radio Scotland.
"We've gone behind at the set-piece, then we hit the bar and they counter-attacked us and score a second, that's just football."
Kilmarnock enjoyed a bright opening without causing too many problems for the home defence.
But the visitors were under the cosh after the interval and were punished when Jelavic peeled away to connect with a Steven Davis and steered a shot beyond Cammy Bell from 12 yards.
Gary Harkins was denied an immediate leveller when his header struck the crossbar and Rangers raced away to add a second goal through Gregg Wylde.
"We looked quite secure, then we don't defend the set-piece," continued Shiels.
"They hadn't really got behind us to threaten us and I thought we'd stifled their imagination.
"We had good composure and we got in behind their lines at times - not as much as we'd have liked because you have to respect the quality that they have."
Kilmarnock played with three central defenders but Shiels was quick to dispel the importance of formations.
"These are only starting positions," he explained. "It's about what's in the player's mind. It's not about 4-4-2 or 4-5-1 or whatever people talk about.
"It's about getting into creative positions. It doesn't matter if you play seven at the back, as long as you attack in units and try to penetrate from there.
"I thought we could have done more of that but there have been some positive things come out of tonight's game for us."