The actress, who plays Jenny Bradley in the soap, has worked closely with Barbara Knox over the years.
Jenny is set to marry Johnny Connor, though filming the wedding left Sally with a few surprise injuries.
But the character actually appeared in Corrie years ago too, when her father Alan wooed Rita.
Now, their relationship is set to be put to the test again when Rita is hospitalised just before Jenny’s wedding.
“It’s a storyline that Barbara is so deserving of”
Sally Ann Matthews
The diagnosis has not yet been revealed, but Sally has revealed viewers will “go crazy”.
Speaking at a Corrie press day, she told Daily Star Online: “It’s a storyline that Barbara is so deserving of.
“It’s been so well thought out and she’s been very involved in the way that it’s going to play out. She’s worked her backside off on it and she’s amazing that she’s 84 and she’s worked such long days.”
She continued: “Barbara’s a fantastic actress, the consummate professional.
“The three of us, with Dolly [Rose Campbell] as well, they’re very easy scenes to do because you’re all in it, you’re all on it.
“The emotional stuff is very easy to play with Barbara. They are going to be tugging at your heartstrings.”
Sally remained tight-lipped on the subject of the diagnosis, but did warn: “Just get lots of Kleenex and wine.”
Rita is hospitalised during the week of the double Connor wedding, leaving Jenny’s ceremony in doubt.
Sally didn’t reveal whether the ceremony would go ahead, instead telling us: “She didn’t notice Johnny had MS and Rita’s not quite been herself.
“But it’s a big wakeup call for Jenny – and what is her priority? It’s quite interesting the direction she goes in.”
Coronation Street airs Mondays, Wednesdays and Fridays on ITV.