Saints finalised a two-month contract for the Bosnia international on Wednesday after Frazer Wright was suspended for three Scottish Premiership games while Steven Anderson waits to get the plaster off his dislocated thumb.
Janic impressed during a trial period but manager Tommy Wright is not prepared to throw him into the starting line-up with his defence at McDiarmid Park on Sunday set to be formed by natural full-backs Gary Miller, Dave Mackay, Brian Easton and Tam Scobbie.
The Saints boss said: "He will go straight into the squad. He signed last weekend but the paperwork didn't go through in time to enable him to play in the under-20s so I think this Sunday is maybe a bit early for him, but it gives us defensive cover.
"It's an area where we are going to be a little light for a week or two.
"We will definitely have four full-backs in our back four but they are all experienced and they have all played in the centre of defence. We will work on it in training and see what the best combination we can come up with for Sunday's game."
The Saints boss believes his 31-year-old signing, who was a free agent after a spell in the Turkish top flight, will be an asset. Jahic has vowed to use his time in Perth to ensure a recall for World Cup finalists Bosnia.
"That's one of the reasons why it was basically a no-brainer to bring him in," Wright said. "He is experienced, he has played at a good level, and he's hungry. He needs to play to get back in the Bosnia squad and we'll give him the opportunity to do that.
"We will look at it at the end of that deal. If he does really well, he might move elsewhere. But the main thing is he's here and he will make a contribution to the squad."
Former Kilmarnock defender Wright was sent off in controversial fashion for denying Paul McGowan a goalscoring opportunity during St Johnstone's 4-3 defeat at St Mirren Park, but he was handed an additional two-game ban after being reported for foul and abusive language towards the fourth official.
His manager, who also lost striker Steven MacLean to a knee injury in Paisley, said: "We haven't appealed it because Frazer has admitted he did say that and that has been dealt with by the club internally.
"I would just like to see a level of consistency because there have been instances in games this year where even from the touchline I've heard foul and abusive language and it's been ignored.
"So it's harsh to take but he is wrong and he knows that and the fourth official had no option.
"Whether he should have been sent off or not is a different debate, but Frazer's reaction is wrong and he knows that and he has apologised."
Motherwell goalkeeper Lee Hollis will miss a second successive week with an ankle injury ruling him out against St Johnstone in Perth.
The 27-year-old missed last weekend's win over Hearts and will be replaced at McDiarmid Park for Sunday's Scottish Premiership fixture by Gunnar Nielsen.
Former Scotland winger James McFadden, meanwhile, is close to a return following a hamstring tear but may not be fit enough to feature.