stuey I’m not sure Watson would be too worried if Milner was still playing for Cas or not but his inside knowledge cold prove useful.
However, the ‘con’ could extend as far as salary payments as loan players are often still paid a fair proportion of their salary by their parent club. This is often why a player goes on loan for the remainder of the season with a deal to sign the following year. The new Club save a fortune on the salary cap. It also makes it difficult for the parent club to use what little space is freed up on the cap.