With City in possession of a seemingly unassailable advantage, Pellegrini made eight changes from the side that beat Cardiff in the Premier League on Saturday.
The impressive depth of City's expensively assembled squad was on full display as the Chilean was still able to field the likes of Negredo, Aguero, Jesus Navas and Fernandinho.
Hammers boss Sam Allardyce handed Andy Carroll his first start since the final game of last season after the England striker's long struggle with a foot injury.
Given the inevitability of City's progress to Wembley, it was hardly surprising to see vast swathes of empty claret and blue seats around Upton Park.
The 20,000 stay-away fans couldn't be blamed and their decision looked even wiser as West Ham's creaky defence was breached with less than three minutes played.
City's teenage Portuguese winger Marcos Lopes had time and space to float a cross into the six-yard box, where Spanish striker Negredo rose completely unmarked to power a header past Jussi Jaaskelainen from close range.
The muted City celebrations suggested even they were slightly embarrassed at the ease of the goal and their path to Wembley.
- AAP