It has got us thinking of deals between the Reds and the Parkhead giants since the 1960s and we came up with SIX names.
We're counting transfers even if a player was out of contract before earning the move but there cannot be any other teams in between the spell at Old Trafford and in Glasgow. Hence, the player's career CV must list Celtic and United (or United and Celtic) together in chronological order.
For this reason, the likes of Dion Dublin and Henrik Larsson do not count. Neither does Scott McGarvey, who was with Celtic's Boys Club.
Permanent deals only - no loans.
The answers are supplied above in the black box. Look out for another trivia poser next Thursday.