The South African scored 18 goals in the Premier League in his debut season at Ewood Park after arriving from Porto in 2006, but has mustered just 19 in the subsequent two and a half years.
And Allardyce believes that, while the 32-year-old will start positively for the Hammers, his counterpart Gianfranco Zola will need to ensure someone picks up McCarthy's workload.
"McCarthy is a hugely talented player who can play his best football in the final third," said Allardyce. "He's not getting any younger, so when you're not getting any younger the legs are not quite as good.
"But the talent is still there, so if you can get other players around him to do the workload then he'll get you chances and score you goals.
"He's generally the best he is when he first goes to any new club.
"When he came to Blackburn he scored 23 goals in his first year and hasn't quite replicated that since."
Source: Team Talk
Source: Team Talk