King featured in Hull's 3-0 win over Bristol City on the weekend, but also played for United reserves last week.
"Joshua's not had a lot of football this season," Joyce told manutd.com. "He's in the situation where the ground has been frosty and hard up there and he's trying to get himself going to play and break into the Hull side. He's not had much football and asked if he could come here and train for a day or so because he knows we've got undersoil heating.
"So we gave him a run-out for 60 minutes on Thursday to try and build him up to help him get in the team there."