Both Roberts and Boyata played for Manchester City before arriving at Parkhead – the winger on an 18-month loan deal and the central defender on a four-year contract.
He said: “It’s a tragic thing to happen and my thoughts go out to all the families. Girls and boys going to watch a concert.
“It wasn’t far from where I was living in Manchester. It is a horrible thing to have happened in the world and my thoughts are with all the families affected.
“I’ve been to the Arena a few times, it is the main arena in Manchester and they have a lot of things on there.
“What a sad thing to witness for the people who were there. It does put life and football in perspective. It’s a tough time for the whole world and it’s good that we can unite and get behind the country.
“It’s a tragedy. As soon as I heard it this morning, I texted everyone that I know back in Manchester to make sure they’re okay. It’s very bad news and all of us are thinking about those affected. It’s so sad.”