It may look a lot like an illegal play but it's not. You can't contact a player below the waist if he is already engaged, but there is no rule against contacting a player above the waist after someone else has tried to undercut him.
I didn't see anything wrong with this. You had a slot receiver trying to undercut Hardy and he didn't get him cleanly so the Memphis O-lineman stepped in and pushed him. I looked at it several times and didn't see the Memphis guy holding his foot, it was just a odd play. Didn't look intentional at all but it sucks for Ole Miss. He didn't look bad hurt, I hope it's not too serious.