Cullen has long been touted as a potential first-team regular at the London Stadium, but his swift exit under Pellegrini suggests that he still has a long, long way to go to achieve that.
The 22-year-old started the season well at The Valley, impressing manager Lee Bowyer so much that he has already admitted he may pursue a permanent deal at the end of the season.
Things have however taken a turn for the worse for Cullen in recent days, with the youngster suffering a serious shoulder dislocation which requires surgery and could put him out for four months, as reported by the South London Press.
That prognosis puts his return date around the end of February or early March, meaning he may only have a small part to play in the rest of Charlton’s season.
While this blow is obviously devastating for Cullen in the short-term, it is the long-term consequences which will likely be far greater for the 22-year-old.
Pellegrini was obviously not too impressed by the 5ft 9in ace [Transfermarkt] in the summer, and so the midfielder really needed to have a stunning season to be in with any chance of breaking through with the Hammers next season.
His early season form suggested he may be on the way to doing exactly that, but this injury now puts paid to his chances of catching the eye for the entirety of the season.
And, in turn, that means that Cullen’s days at West Ham are now likely to be numbered, with their being little chance he can impress Pellegrini enough to warrant a first-team place in the summer.
It would be a cruel, cruel blow for the talented young midfielder if that were to happen, but it is clear he has the talent to succeed at a decent level in the future.