It is way too early to drop these guys. Those who own them might be panicky and many of these players might be excellent buy-lows. But for now, all that is known is they had horrendous Aprils and if you are an owner at this point, you won't get fair trade value, so you're best sitting tight.
That being said, it's one month—162 games makes for a long season, and if one of these guys had a slump in July, no one would be worried. Be patient with your stars and don't fall victim to a facepalm-type trade.
On the other hand, one of these guys could be the new Adam Dunn, so owner beware.