The Scotland Under-21 international midfielder Ryan Gauld, nicknamed “Baby-Messi” by team-mates, has completed a remarkable transfer from Dundee United to Sporting Lisbon for an undisclosed fee.

The 5ft 6in 18-year-old, who made his United debut in May 2012, has agreed a six-year deal with the Portuguese club, with the contract including a €60m buy-out clause.

United confirmed in a short statement that the move remained subject “to the usual formalities”, but that “everyone at the club wishes Ryan sincere best wishes in his future career”.

Speaking before the move was confirmed, United’s manager Jackie McNamara told STV: “There comes a time when it’s too good to turn down, not just financially but for the player himself. If it’s the right club it could be life changing.”

Gauld, a former Dundee United ball-boy, told the Observer last year why he felt a move abroad might suit his style and attributes better than joining a major club in England.

“When I’ve been growing up, I watched the Spanish leagues and prefer the style of that. Getting the ball down and being patient with the buildup play. In England, it’s maybe more direct. If you are a winger you are expected to take on a full-back and get a cross in. If it’s abroad, you can turn out, keep the ball and just keep the attack going.

“Abroad you see more players who are smaller in stature. In England, it’s more athletes. Me being a smaller guy, I need to think about what’s best for me and what’s best around me and I think being abroad is the best culture of football for me.”

McNamara told the BBC in December he expected Gauld to go on to become a “British great”.

“He’s going to be a massive star in our national team, there’s no doubt about that. I have been fortunate in my career to be working with some really good young players … but, for me, Ryan, you look at him and he’s still like a wee boy, his size there, but he flies into tackles, he goes for headers, and he’s only going to get stronger and quicker.

“Everybody can see the talent the boy has. For me, he’s one of these boys who will always strive to be better. His vision, his ability is just fantastic. Upstairs, his football brain is way beyond his years, way beyond anything I’ve seen.”