Saturday taught us that Mousset will struggle if playing upfront on his own. It's not that he doesn't work hard, it's more that he needs to be spurred on to chase down loose balls and harry defenders by a partner, a la vs Chelsea with Wilson.
Regarding Rico...he's a quality player. He looks like he's trying too hard atm, looks a bit panicky on the ball and while defending. Add that to the new culture, new language, new team dilemma, then its reasonable to see why he might be struggling; even well into January and beyond.
Lerma seems like a genuinely intelligent guy in general, which might explain why he's settled in quicker. He seemed to get his role and what he needed to almost immediately. Just look at how he adapted his play vs Watford so as to not get as involved in all the fouls.