The former Bradford City chief had only joined previous club Blackpool in the summer - as number two to Neil Critchley - having been out of football since leaving the Bantams last December.
Now he finds himself back at another former stomping ground in Yorkshire in Bramall Lane, having linked up with Blades manager Paul Heckingbottom, who served as a player under him during his first spell in charge of City.
McCall said: "Hecky called me out of the blue last week at some stage and we just had a little chat. He just said: "In the future, can I see yourself working (with him).' But he knew I had just gone in at Blackpool and was enjoying myself - I'd only been in four months working with good staff and a good manager and good group of players.
"I said: 'It might be something that is in the future.' Thinking that the future was six months, 12 months or two years or whatever it is down the line.
"Then on Wednesday afternoon, I got a phone call out of the blue saying that there is a possibility that there is going to be a change being made. Would you be able to talk?
"At that stage, I got quite excited. I had four really good months at Blackpool. People know I am a loyal person and for one second, I was not looking to come out of the club or anything. This has come right out of the blue.
"There were a lot of things obviously. I know the club really well and thoroughly enjoyed my years here. I know Hecky well and it is obviously a lot nearer home and there's the size of the club and the actual squad of players.
"We came with Blackpool a couple of weeks earlier and after seventy minutes, Sheffield United played us off the park, really and should have been ahead. We knew we'd nicked a result."