The summer transfer window in Scotland closed on Saturday, and Killie failed to bolster their ranks before the deadline.
The only late movement they saw involved Paul Heffernan, with the striker heading to Hibernian on the eve of a 2-1 defeat to Motherwell.
While no more money can be spent before the New Year by Scottish Premiership clubs, Johnston is planning on dipping into the free agent pool for a couple of fresh faces.
He said in the Sunday Mail: "There are still players available who are out of contract.
"Heffernan was the top-paid player at the club and his leaving gives us a chance to bring in a couple of players.
"We need two more, including a centre-half, and you can see we will have a decent squad.
"We've been trying to bring in a centre-half for a while.
"A couple we were trying to bring in on loan fell through as they ended up playing first team. That was last-minute and frustrating."