Kelly answered the call from Unsworth after he was put in charge following the sacking of Ronald Koeman.
The former goalkeeper worked with Unsworth when the pair were at Preston North End together and says when the call came he needed no time to say yes – even if he was on a family holiday.
“I was sat in the Guinness factory in Dublin, going for the experience and the culture, of course!" Kelly told the Liverpool Echo.
“Then I got a call late in the evening with the words: 'Please tell me you're in the UK?'
“I wasn't, so it was a bit like the film 'Planes, Trains and Automobiles' to get here.
“I jumped on the last flight, jumped on a train and jumped in a taxi to get home in the early hours of the next morning, then I was up and ready to start at USM Finch Farm for 8am.
“It really was a whirlwind.
“But, at the same time, it was a very easy decision. If I could have water-skied across I would have done!
“My relationship with Unsy is a long-standing one and a very good one.
“The best way I can sum it up is – the call came and I answered. You don't do that unless you want to work with someone."