Of course you can't sub on the fly like "sunday league soccer". But Silva COULD have run out to the touchline to swap places with Maund. If Silva was taking too long, the ref could have booked him. But there is nothing in the rules stating that Silva couldn't have walked/jogged towards teh half way line to be replaced.
I have no idea why Silva went straight behind the goal! TFC were leading at the time, they needed to WASTE time, not speed it up!