Killie sit eighth in the SPL table, four points off sixth spot, with another five fixtures before the split.
However, Shiels has been concerned by his team's recent form, and called on his troops to stand up and be counted.
He said: "If we don't win tomorrow, then I think that would more or less be it.
"We're four points away, which seems like quite a lot, especially when there are only five games to go.
"It won't be an easy match, either. St Johnstone have made some good investments, and Steve Lomas has done really well since taking over as boss from Derek McInnes.
"I just hope we can dig in, and manage to get a result."
Dean Shiels played 90 minutes for Northern Ireland on Wednesday, in their 3-0 defeat to Norway.
The 27-year-old – son of the Killie boss – also picked up the man-of-the-match award.
Shiels said: "I thought he performed really well. It was his first involvement under the new management team, so I'm pleased for him.
"His award is a good omen ahead of our next match."
Lewis Toshney and Liam Kelly are in contention for a return to the side after overcoming injury and suspension woes.
Meanwhile, Danny Buijs and Gary Harkins are also on the road to recovery.