I checked the DC rules for 2014. They allow 5 players (if the captain is non-playing) on roster. So if Shamasdin was included in the 5 players roster for the tie, he could replace any players on the draw for doubles and reverse singles.
Ok, I checked the rulebook again today because I remember this is not the case last year and recognized I interpret it wrong.
Under team nomination, 10 days before the commencement of the tie, following should be nominated by each team.
A team of a minimum of three and a maximum of four which shall include the nomination of a playing Captain; or
A team of a minimum of four and a maximum of five which shall include the nomination of a non-playing Captain
Since Canada's captain is not a player, the team can have a maximum of four players.
Before the draw, the four players were Raonic, Pospisil, Dancevic and Nestor.
The rules allow two of the nominated players to be changed up to one hour before the draw and that's when Canada replaced Raonic with Polansky.
Since the four players nominated did not include Shamasdin, team Canada could only replace Pospisil by either Dancevic or Polansky.