Hernandez's record is good, roughly 1 goal every 3 appearances, considering many of those are 20-30 min appearances off the bench it's probably a very good goals to minutes conversion rate.
Costa has had 18 months at hiscurrent level and spent a lot of last season injured, Aguero is injury prone too. Rooney is fairly solid and rarely misses many games and has had to play a large chunk of the last 2 seasons out of position. Play either Costa or Aguero in midfield for extended periods see how well they do.
RVP was injured most of last season, Falcao was having a nightmare and Rooney as mentioned was mostly in midfield no surprise none of our players broke the 20 goal mark.