Barry Robson will return to Tannadice tomorrow night in the hope of completing a remarkable transformation. Against all expectation, not least his manager's, the £1.2m January signing has been arguably the most significant contributor to Celtic's rousing finale.

Robson's inclusion in both Old Firm derbies at Celtic Park tipped the balance in favour of the defending champions. Where Scott Brown and Massimo Donati floundered, Robson, a comparative transfer bargain, has flourished.

Gordon Strachan's decision to unleash the strapping, flame-haired midfielder coincided with an upturn in performance and results. Yet it was more by accident than design, with Strachan finally giving up hope of fusing the aforementioned box-office signings into a formidable midfield partnership.

Robson's presence has added aggression to a midfield that had become passive. It is a meteoric rise for a player originally signed as cover, and perhaps as a long-term replacement, for Shunsuke Nakamura.

His prominence has not surprised one of his early coaching influences.

Ian McCall spent £50,000 taking Robson's raw materials to Dundee United from Inverness Caledonian Thistle and helped plane him into a smooth operator.

His success at Celtic, McCall insists, is merely reward for Robson's dedication. "I remember a good few friends in the media slagging me off for spending £50,000 on him at the time," said McCall. "I think he is a great example of somebody who maybe came off the rails a bit when he was younger but who has since put everything into his training and games.

"I always thought he was destined for a big move like this. In fact, when I was at United we had interest from the Premiership but he was still developing. It was only a matter of time. At the time, I thought he was the best in Scotland at set-pieces, maybe along with Alan Thompson.

"He had this fabulous ability and a real belief in himself: not arrogance, just pure self-belief. I think he would have fully expected to play regularly for Celtic before he did. No matter who is in the team, he looks like he is going to be a fixture now."

The most significant of his three goals was the penalty kick confidently dispatched beyond Neil Alexander to secure a 3-2 win against Rangers in the final installment of the Old Firm franchise. It was the moment he earned his spurs as a Celtic player, unceremoniously assuming the responsibility from Scott McDonald in the discomfiting din.

"He is a right moody bastard in training and on the pitch," said McCall. "That's what gives him his edge.

"He maybe does not have the electric pace that separates the real top-class players but he has more than made up for that. The first thing that struck me about him was his size. It doesn't look like it but he has a massive build and really big shoulders. It gives him his strength and power."

His deliveries have been an even richer source of plunder; his vicious, inswinging corners and free kicks suddenly exploiting the latent target-man qualities of Jan Vennegoor of Hesselink and Georgios Samaras.

Robson has also spawned a fertile bar-room debate: does he possess a more devastating control of a dead ball - corner kicks and free kicks - than Nakamura? McCall believes he is the best all-rounder at Strachan's disposal.

"His game has improved no end," he said. "He has the skills of a wide player when going forward, he can pass the ball across the midfield and, when he tracks back, he is physically capable of playing in there. It is a potent mix. He has a lot of praise already from the Celtic supporters and the fans and, if you win over the fans, then you are halfway there.

"I signed him as a wide player but had to work hard on him," added McCall. "When he was with Caley Thistle, they were shooting up the divisions and didn't really need to worry about defence too much. They were never troubled enough for him to have to come back across the halfway line, so we had to work hard on that part of his game when he arrived.

"He did it without complaint and it has made him a more rounded player and enabled him to play in the central role he now finds himself in at Celtic."

His attributes are only now receiving a wider recognition, but his 15 goals in the first half of the season were sufficient to merit inclusion in the PFA Scotland shortlist for player of the year. Already he has scored his first derby goal, and signalled his arrival in the Champions League with a goal against Barcelona. McCall confidently predicts a prosperous international career under George Burley.

"Paul Hartley was another late developer and cannot go on forever," said McCall. "His replacement might be beside him at Celtic. I think he Robson would fit beside Barry Ferguson and Darren Fletcher because you then have a great balance, with Robson being predominantly left footed."

For now, Robson will concentrate his efforts on dragging Celtic across the finishing line against United. Strachan is grateful he indulged his curiosity in the City of Discovery.