Speaking to watfordfc.com, Mackay said he wouldn't be placing "huge expectations" on the slight shoulders of the player, who he says is at Vicarage Road to learn.
"Ryan is a young prospect at Sunderland and we felt that he was the kind of player that could add to our group from now until the end of the season," he explained.
"He is young and he's here for experience more than anything else, we will get him involved in the team and he's an extra face to have around the squad to make sure we have enough cover here.
"Our scouts had been looking at him over a period of time, but this is his first step out of Sunderland's academy, so I don't have any huge expectations on his shoulders.
"He's a young talent, who Sunderland feel will do well for them in the future and he's come down here for a bit of experience."