Manager Derek McInnes preferred the aerial threat and physical strength of Jayden Stockley from the start last night but as has so often been the case, if Rooney isn’t scoring then the Dons aren’t scoring.
There was certainly bags of passion and aggression from both teams but it was left to Aberdeen’s most reliable striker to produce the moment of subtlety needed to decide a match high on endevour but low on magic moments.
Rooney replaced Stockley in 72 minutes and hardly had a sniff of a chance until the dying embers when Graeme Shinnie worked space down the left and flighted a cross that marksman adroitly steered.
It was Rooney who scored the crucial penalty in the shoot out when the Dons defeated Inverness Caley Thistle in the final of this competition two years ago when they beat St Johnstone in the previous round.
Now they are just one more win away from another final after breaching the visitor’s makeshift defence to leave the Perth club with just one win in 21 knockout ties between the teams.
Former Hearts youngster Liam Gordon was drafted in for only his second game for St Johnstone to partner Steven Anderson in central defence in an effort to stem a formidable Aberdeen attacking line-up.
One player he must have been surprised not to face though was Jonny Hayes, named in the starting X1 but forced to pull out after suffering a hamstring injury in the warm-up that will surely mean missing Sunday’s eagerly anticipated visit from Rangers as well.
Mind you, Niall McGinn’s promotion from the bench could hardly be described as weakening the team and the Northern Ireland international’s pace was a constant threat as Aberdeen dominated vast swathes of play.
That certainly made it a testing time for Gordon in particular but someone on loan at Elgin City last season certainly didn’t look out of place, making a crucial to deny Graeme Shinnie an early goal.
However he did suffer the indignity of being nutmegged by James Maddison, the midfielder on loan from Norwich City who has earned instant hero status with the Aberdeen supporters for his control and touch on the ball.
The teenager was the fulcrum for most of the good things the home side did in an opening period during which St Johnstone struggled to get out of their own half and certainly carried little goal threat.
Maddison, and the Dons most incisive moment during that time came virtually on the half time whistle at the end of a fiercely contested period as a sweetly judged reveres pass sent McGinn scurrying clear down the left flank.
His cross on the run was curled invitingly into the path of Burns but under pressure the striker on loan from Bristol City hooked the ball wide of Zander Clark’s goal.
Tommy Wright was forced into a change early in the second half when striker Joe Gormley limped off injured to be replaced by Michael Coulson and the introduction of the winger along with the change of shape that brought saw Saints play more of the game in the opposition half.
That much was clear when Danny Swanson finally managed to wriggle clear, cut in from the left and curl one of his trademark dipping shots that Joe Lewis in the Aberdeen goal was happy to see narrowly clear the crossbar.
It all led to a game that became progressively more stretched the longer it wore on, although all the effort and a crime count of seven bookings, five for visiting players, neither goalkeeper was seriously troubled until Zander Clark saved a Kenny McLean free kick in 82 minutes.
McLean should certainly have given Clark something to think about earlier when he eased onto a Maddison corner but the midfielder’s volley from just six yards range flew high and wide.
At the other end Lewis was finally forced into serious action with just three minutes to go, brilliantly tipping David Wotherspoon’s 30 yard free kick on to the post at full stretch.
Just how important that turned out to be came just when the match seemed destined for extra time and of course it just had to be Rooney who supplied it with a glancing header from Shinnie’s clever cross.