I wouldn't be surprised if Chadli being on was for some physical presence against Smalling. Lennon played well in the first half and I think we clearly targeted Evra as our avenue to break through, meaning the left side needed to be kept solid and locked down. So we keep Chadli on when Lennon comes on because we want Evra to be facing a constant stream of pace with fresh wingers who are able to go full pelt. It almost worked, Andros was a toe away from scoring and Defoe exploited space down their left side before getting pulled back by Vidic in a great position.
In terms of a plan for the match, ensuring Evra is up against 2 fresh wingers playing at full intensity is a sound proposition. And considering Lennon busted a gut for most of the first half and didn't do much at the start of the second, it again wasn't a terrible idea.