The 34-year-old is out of contract and on the hunt for a new club after leaving Huddersfield last summer.
Over the past few months, he has been training at City – and at England headquarters St George's Park – to keep up his fitness, and was working alongside another title-winner in Andy King, who has since joined OH Leuven.
Then, on Monday night, Simpson was drafted into an inexperienced Under-23s side with plenty of players from the development squad training with the first team ahead of Wednesday night's FA Cup tie with Brighton, manager Brendan Rodgers calling up some of the club's youngsters for the clash.
Foxes boss Brendan Rodgers said, "Danny is someone who can come in, he's obviously not attached to a club at the moment and not blocking the path of any young player.
"I think it's a really good opportunity for him to come in and get some game-time, but also for our younger players to learn from somebody of that experience.
"That is sometimes what's missing now in Under-23 football, where in days gone by younger players were playing alongside experienced players who have achieved things and done things in the game who you can learn from.
"You don't get that so much now and that can be important for younger players. Danny is a really good guy, he can help us and we can help him back to fitness."