I think this is his 3rd or 4th appearances so far.
Maybe I need to see more games, but I just can't see anything useful which he can provide to the first team at all. I mean before him, the likes of McT, Tuanzebe, Gomes, Greenwood, Chong, Rashford, Andreas Pereira (or even TFM, Mitchell, Wilson, Januzaj or Harlop even if for these five, unfavorable developments afterwards) all showed something immediately from their first debut -- something useful they can bring to the table for the first team to use or which the first team lacking. None so far with Garner. A fair number of those youth players before him also played very small number of minutes, so it shouldn't be a case of lack of chances.
Perhaps next games, he can finally show it.