Due to the new arrivals Conway, who handed in a transfer request last season, will now be allowed to leave the club on-loan as revealed by Mackay.
"It was difficult to pick the squad and trim things down, but we did that as Premier League rules state we must," Mackay said.
"It was unfortunate for Craig, but he has always said that he desperately wants to be playing football.
"When he came to me about the transfer request last season that was very much what our conversation was about.
"That is fine and I totally understand that, but obviously I could not give him any assurances about that at this club.
"With the signing of Peter Odemwingie, Craig's chances were inevitably going to be limited."
For the big stories and the banter follow us on Twitter: @tribalfootball