He made his debut at 15 for Argentinos Juniors, but he wasn't playing for that team beginning with 'B' in Europe at 17. He didn't leave Argentinos Juniors for Boca Juniors until he was 20 as well...........which also uncannily happens to be the same age Mac Allister did.
Written in the stars...
There was another 21 year old Argentine No.10, who played for Argentinos Juniors, then Boca Juniors, before going to Europe to play for a club beginning with the letter 'B', who had a surname beginning with 'M' as well. It's written in the stars.
Does he have the ancestry/grandparents etc? Looking on wikipedia his Dad is described of ' Scottish and Irish descent'.
https://en.wikipedia.org/wiki/Carlos_Mac_Allister
Perhaps we wouldn't be bothering at all if it was an issue though.