United were thought to be weighing up a £60 million ($74m) bid for Maddison back in June, having been impressed with his performances at the King Power Stadium throughout the 2018-19 campaign.
The 22-year-old contributed seven goals and seven assists in 36 Premier League appearances for the Foxes, helping the team finish ninth in the final standings.
Maddison has transferred that form into Leicester's latest season and been rewarded for his efforts with a call-up to Gareth Southgate's senior England squad.
The attacking midfielder was left on the bench during the Three Lions' 4-0 win over Bulgaria at Wembley on Saturday, but he could be in line for his debut against Kosovo on Tuesday night.
Leicester legend Steve Walsh has been among those to insist a prized asset will be staying put, but as his stock continues to rise, so to do rumours over his position at the club.
Maddison has previously remained coy on his long-term future, but has now admitted that he is driven by an ambition to reach the very top of the game in the coming years.
"I want to play at the highest level my ability will allow me to," he told the Mirror.
"I like to think I’ve my head screwed on with the transfers I’ve made. I’ve played League One, in Scotland, the Championship and the Premier League. I worked my way up."
After returning to the King Power from international duty, Maddison will prepare with the rest of the Leicester squad for a huge clash against United at Old Trafford.
The Foxes sit third in the Premier League table after four matches, three points ahead of the Manchester outfit, who have only picked up one win.
Maddison has yet to find the net this term, but he has laid on two assists and produced a standout display in a 1-1 draw against Chelsea at Stamford Bridge.
Brendan Rodgers' side have been tipped to break up the top six monopoly come next May, despite having already sold one prized asset to the Red Devils during the summer.
Harry Maguire joined United for £80 million ($99m) at the start of August, becoming the most expensive defender in history.
He will likely feature against his former club next Saturday, having been a fixture in Ole Gunnar Solskjaer's line up since his arrival in Manchester.