Fish is a tall centre-back who has risen through the ranks at the club. Although still eligible for the Under-18s, he has been a regular in the Premier League 2 side this term under lead coach Neil Wood.
The 18-year-old has also appeared in this year's FA Youth Cup and will relish the experience of being around Ole Gunnar Solskjaer's senior squad.
Formerly Under-16s captain, he has taken on the armband at a higher level at times this season with the Under-23s and displays clear leadership qualities from the heart of the defence.
Under-23s team-mate Anthony Elanga was added to the B list for the Europa League ahead of the first leg in Spain, and was on the bench, with Fish the latest new name on the roster.