JUST six months ago the notion that Sam Cosgrove would be regarded as a key player for Aberdeen on the biggest occasions would have been regarded as preposterous.

The 21-year-old arrived without much in the way of fanfare from English League Two side Carlisle United in January last year, having harvested a solitary goal from 12 appearances for the perennial

fourth-tier strugglers.

He was then ordered off on his debut for the Pittodrie club after a studs-up challenge on Celtic captain Scott Brown just eight minutes after coming on for Stevie May.

Cosgrove failed to find the net in his first 16 outings for Derek McInnes’ side, by which time he was being compared unfavourably to his predecessor, Jayden Stockley, who has since moved to the English

Championship side Preston North End.

However, since breaking his duck against St Mirren in October, he has scored 20 times in 28 games and former Celtic, Aberdeen and

Scotland midfielder Paul Hartley believes he is the man who can do more than anyone else to put a spoke in the wheels of Celtic’s triple treble tilt.

“This time last year most people thought that he wasn’t going to cut the mustard because, at that time, he couldn’t buy a goal,” Hartley

said. “But he was still a young kid when he came from Carlisle United and he had to adjust to the Scottish game.

“This season he’s shown he’s

more than a handful by scoring all types of goals. He’s been really impressive.

“People talk about the physical presence of Uche Ikpeazu but Cosgrove has that as well, plus he has height and pace. He also has a good first touch and links up well with his team-mates but the most important factor is that he knows where the goals are.

“The hardest job in football is

scoring goals and I’m sure he’ll be the main threat for Aberdeen, who are a younger team this season.”

For years Aberdeen had been branded big-game bottlers and their supporters had grown tired of the long journey back to the north-east following another disappointment in Glasgow but McInnes appears to have solved the problems they faced at the three biggest grounds in the country.

“I think they have a great opport-unity,” said Hartley. “Derek will look at how they comfortably disposed of Rangers at Ibrox in the quarter-finals. Most people probably thought their chance had gone when

they drew the first game at

Pittodrie but they’ve shown they’re more than a match for Rangers and Celtic.

“They beat Celtic at Parkhead at the end of last season and got a draw there a few weeks ago. They’ve also beaten Rangers three times in

Glasgow this season so Sunday’s game won’t faze the manager or the players.

“On top of that, they’ve become accustomed to playing at Hampden over the last few seasons, even if they haven’t cleared the final hurdle against Celtic.

“Obviously, Aberdeen will be

missing a key player in Graeme

Shinnie but they still have enough quality to make them believe they can win.

“Celtic haven’t been firing on all cylinders but they’re accustomed to coming to the national stadium and winning.”

Hartley, who won the Scottish Cup with Hearts in 2006 and Celtic in 2007, though, contends that the fact the game is in Glasgow will work in Aberdeen’s favour.

“Their main problem has been their home form,” he said. “They’ve won just two out of nine at Pittodrie this year and one of them was against Queen of the South.

“For some reason they seem to play better away from home so they’ll come to Glasgow knowing that their away form is excellent – nine wins and a draw in their last 11. So they’ll know that they can put on a good show against Celtic.”