City's outlay has contributed to one Premier League title and two League Cup wins, while United have claimed FA Cup League Cup and Europa League success over the same period, yet both clubs may have expected more after such vast outlays.
City ended third in the Premier League after a fine start under Pep Guardiola petered out in an ultimately trophyless campaign.
"Man City have spent the biggest amount in the Premier League", Herrera stated. "But I think because of the money they have spent they are the favourites".
"It's normal for United and City to be competitive and to fight for the title and the other competitions".
Despite having won just one of its three league home games, Chelsea has been ideal on the road and stays within three points of the Manchester sides at the top of the table.
"So they are the principal contenders for the title".
Arsenal are due announce their latest set of financial figures in the coming weeks, with supporters unlikely to be impressed when they gather for the annual AGM, as these figures suggest Wenger's side are underperforming given their net spend over the last five years.
"But you know when you play for Man United wining is an expectation and an obligation", Herrera added.
"In the second half they had a couple of chances but the best keeper in the world (David de Gea) was there for us".
Having finished 10th in 2015/16, Chelsea nearly came from nowhere to claim the title in record breaking fashion last season.
"Of course I can see him staying here for more years". He's got damage to his hamstring and we're hoping it won't stop him playing, but clearly at the moment he's a big risk.
Stoke increased the pressure after the break, but former Real Madrid striker Morata doubled his tally with a brilliant solo goal to end home hopes of a revival.