United are still scrambling to sign a central defender before the transfer deadline on Thursday, and have been strongly linked with Leicester's Harry Maguire.
A move for the England international looks difficult at this stage, but Merson still feels that United will get their season underway with three points at Old Trafford.
"This will be a good game. Pressure is on at United after not a great pre-season," Merson told Sky Sports News.
"This should be perfect for Leicester as United will have a go and they can be caught on the counter-attack, but they'll miss Riyad Mahrez. I just think Mourinho will find a way to get off to a winning start."
United recorded a 2-0 win over Leicester when they locked horns at Old Trafford in the early stages of last season.