The Royle Family star joined the ITV1 soap last month as Stella Price's mother Gloria.
However, the actress became sick two weeks ago just days after she started filming scenes. She was unable to work, prompting rewrites, according to the Sunday Mirror.
"It would be a lie to say this hasn't caused a real problem. A significant amount of time was going to be dedicated to her introductory scenes," a source said.
"We had to completely change the filming schedule. And scenes that had been written had to be changed to allow for her absence."
Johnston's character will shock daughter Stella (Michelle Collins) by turning up out of the blue after years running a bar in Spain.