If I want to go retro and play a classic 4-4-2, I can see Bobby Firmino being the perfect secondary striker foil for another centre forward.
I'm going to call upon 'The Yak' to be my backup #9.
Yakubu Ayegbeni (Nigeria) was a goalscorer with different qualities to Bobby: a 6 foot battering ram, with power and pace, a prolific finisher in front of goal. Scoring 15+ goals a season for the likes of Portsmouth, Middlesbrough, Everton and Blackburn Rovers is an impressive feat. A very respectable total haul of 95 goals (moreover an excellent games to goal ratio for a non-top 4 club striker).
"Feed the Yak and he will score!" 
Firmino - -Yakubu
Hazard- - Lampard -Hamann - -Bale