MacDonald signed his deal after his only league appearance of the season as a sub against Brighton last Saturday where he set up the only goal of the game for Febian Brandy.
Speaking on the official site, Roberto said "It's been a very important campaign for Shaun, even though he hasn't been in the limelight like some of the other players
"He's made huge progress behind the scenes; development which was clearly evident in the last 20 minutes he played at Brighton on Saturday.
"He's worked really hard on a daily basis to improve his technique and strength. He's matured and his body is finally ready to cope with the demands of professional football on a regular basis.
"I think people forget sometimes that he is still only 19. He has a bright future ahead of him and we are delighted that future will be with Swansea City.''
There was also confirmation that Alan Tate has signed a similar deal (performance related) "Alan is one of those players who has been through every emotion possible with the club and is part of a core of players who know exactly what it takes to play for Swansea City,'' added Roberto.
"He's not only versatile, but he's got a winning mentality and never gets affected by any occasion in football.
"He's worked very hard to play Championship football and we are delighted he'll be doing that with Swansea City.''
This article first appeared on JACKARMY.net.