The timing is a little surprising, but it isn't really surprising that a move of this sort was made. I'd have expected them to wait another game or 2, though.
If Boeser and Joshua can both play then we've got, at forward (putting Joshua down the lineup to ease him in, though the lines will undoubtedly get changed):
DeBrusk-Miller-Boeser
Suter-Pettersson-Garland
Heinen-Blueger-Sherwood
Joshua-Raty-Hoglander
Bains and Aman, both of whom have some trust from Tocchet, are available for the 13th forward spot and in case of further injury they could go down the depth chart a bit further to PDG (if and when he is healthy), Karlsson, Sasson or, if they think he is ready or it best for his development, Lekkerimaki. All of them carry a cap hit lower than Sprong. The Canucks were at 48 of 50 active contracts (plus two sliding that won't count) so they gain a little cap space, a roster spot for flexibility for future moves and room to play younger players that may be deemed ready to play.
The only puzzling thing to me is the timing, in that it reduces the organizational depth at a time when Boeser could be out and Joshua isn't back yet. It wouldn't be surprising if, as some have been suggesting, there is another move in the works.