Willian squandered a good chance early in the second half, shooting wide of the far post, and Chelsea were made to pay when Marko Arnautovic set up Hernandez to score only a few minutes after the Mexican striker had come on.
Chelsea FC are in fifth place in the Premier League table and eight points adrift of fourth-placed Tottenham.
West Ham, now six points clear of the relegation zone, were ecstatic to escape with a point despite a largely limp display. While it has a lot to live up to surpass the excitement levels provided in Manchester, it's a game which both teams desperately need to win to restore some lost face.
The 29-year-old played well again today, as he zipped in and out between West Ham's centre-backs, but it was not enough to help his side defeat their London rivals.
Alvaro Morata saw goals ruled out in either half for offside, while Willian missed a host of glorious openings as the Blues failed to add to Cesar Azpilicueta's first-half opener and put the game beyond their opponents' reach. We begged for the goal, ' he told Sky Sports after the match.
His goal had goalkeeper Joe Hart, himself a star on Sunday, praising the diminutive striker for his desire following an global call-up on the other side of the ocean.
Moyes went to his bench and brought on Hernandez, who took just three minutes to make his mark.
West Ham's struggles have come as a surprise to the Cassino-born player, although he admits the squad David Moyes is working with is probably too thin.
WWE WrestleMania 34: Predicting Most Likely Swerves
When this match was announced, I expected there to be plenty of old faces turning up to put on a little show at WrestleMania . The two other teams will try to take them out, but there will no doubt be something that gets in the way.
USA job creation tumbles in March; unemployment steady at 4.1%
The figure was slower than previous months and well below expectations that left the unemployment rate unchanged at 4.1 percent. But retail businesses shed 4,000 jobs and construction companies lost 15,000 workers after going on a hiring spree in February.
Stocks May Pull Back Sharply On Renewed Trade War Concerns
The products on the USA list include heavy machinery, motorcycles and high-tech items, such as semiconductors and batteries. Germany's benchmark 10-year bond yield dipped back below 0.50 percent and toward 2-1/2 month lows hit last week.
BT Market Podcast: RBI policy powers Sensex, Nifty to new highs
The NSE Nifty, after shuttling between 10,331.80 and 10,227.45, finally settled 196.75 points, or 1.94 points higher at 10,325.15. A day after Sensex surged almost 600 points, investors traded cautiously as they looked to book profits ahead of the weekend.
Anti-India protests erupt in Kashmir amid deadly fighting
According to a Kashmir Media Service report, search operations were conducted in Islamabad and Shopian districts of South Kashmir. The Indian police also raided the residence of APHC leader, Muhammad Yousuf Naqash, in Srinagar and harassed his family members.