The German centre-back stepped up his recovery from ankle surgery in the summer with a 64-minute runout at Belvoir Drive.
But two minutes before he was subbed off Demi Mitchell netted the only goal with a fantastic free-kick.
George Thomas, a summer signing from Coventry City, had Leicester's best chance to level, but he headed straight at Kieran O'Hara from close-range.
Man Utd next face Swansea City at Leigh Sports Village on 21 August. Three days before that, Leicester take on title holders Everton at Goodison Park.
See: Leicester report
(Photo at the top of the page courtesy of Plumb Images/LCFC)