Former Hammers favourite Frank Lampard was then also pelted with coins before missing the spot kick.
Kezman made light of the incident afterwards, saying: "It was nothing, just a small accident."
And Chelsea's assistant manager Steve Clarke said: "He is OK and didn't need stitches."
However, the FA were today waiting for referee Andy D'Urso's report and look certain to launch a full investigation.
At least two plastic water bottles were also thrown onto the pitch during the match when West Ham fans targeted their former hero Joe Cole.
Violence flared again after the final whistle when West Ham's fans in the Matthew Harding stand's lower tier fought with police as Chelsea fans in the top tier appeared to throw rubbish from above. It took 15 minutes for order to be restored.
Rival fans then clashed outside Fulham Broadway Tube station as they headed home. Police made 11 arrests for public order offences, carrying offensive weapons and possessing drugs.
West Ham manager Alan Pardew said: "What happened is a shame. There had been some good English banter between the two sides. There is a big rivalry behind these two clubs and both sets of fans were in good voice." A Met Police spokesman said: "The majority of fans behaved well."
The FA are already investigating the crowd trouble in the Millwall versus Liverpool Carling Cup tie at The New Den on Tuesday night. Millwall fans had allegedly taunted the away supporters by chanting " Hillsborough" and "You should have all died at Hillsborough".
They were also said to have chanted "There's only one Boris Johnson", a reference to the Conservative MP who was last week forced to apologise for publishing comments about Liverpudlians "hooked on grief" and their role in the Hillsborough disaster, which claimed 96 lives.
The Millwall fans were also said to have taunted Liverpool supporters about the death of DJ John Peel, originally from Liverpool.
