Sagan's issue with MSR is the same issue that Gilbert has. It's a lottery. Most riders who start are capable of winning it depending on how the race plays out. Remember MSR is the only monument that Gilbert hasn't won. Can Sagan win it, yes, but things have to play out correctly for him.
Valverde has won LBL 4 times, but doesn't have a "favorite" monument. He doesn't like MSR, has no interest in ever racing Paris-Roubiax, and likely won't race Flanders again. His only comment about Lombardia is that is along with the Olympics are about the only things missing from his palmares. The only 1 day race Valverde ever had an obsession with was the Worlds and that was the one race he really wanted. So I'm not sure what you're referring to.