Harrison training with the team possibly signals that current back up Thorne Holder was injured during the Union's 2-1 Reserve League loss to the Columbus Crew. Multiple reports from fans that attended the game have pointed to a shot that Holder took to the head during the game's first half, close to halftime.
Zman Gunner of Big Soccer had this to say about the play:
Holder got nailed with the head with a rocket of a shot shortly before half-time. He finished the half but then made way for Zack Steffen to play the 2nd half. It would not be surprising if Holder at least suffered a mild concussion... Prior to the head shot, the sun may have been troubling him as Holder had a few instances in which he had catchable crosses or high shots (or ones which should have been punched) that he instead just swatted with his open hands.
Concuring with Gunner, but adding new information as well, is @Olmy, who informed the Brotherly Game through Twitter that the Union's medical staff was at least worried about Holder suffering his second concussion of the season.
He took a shot\save to the head in the first half and during the 2nd half they were doing a concussion test on the side. Seemed like he passed it but was still squinting on the bench.
Due to the goalkeeping situation the team currently has, the Union can sign Harrison - or another goalkeeper in the MLS goalkeeper pool - as an extreme hardship call-up.