It condemned Di Matteo, who spent two years as Albion boss, and his side to a first away Barclays Premier League defeat of the campaign.
The Baggies took the lead via their first worthwhile attack after nine minutes. Zoltan Gera played the ball back to James Morrison whose floated cross picked out Long who got in behind David Luiz to head home from close range.
Long was then just off target with a curling drive after he had shaken off the challenge of Cahill but after 38 minutes Hazard levelled for the Blues with his fourth goal of the season. He attacked Cesar Azpilicueta's deep cross and was able to squeeze the ball past Myhill with a powerful header.
Chelsea continued to dominate possession and Victor Moses fired over but Odemwingie restored Albion's lead five minutes into the second half.
This time Long was the creator with a cross to the near post and the Nigeria international got in front of Ryan Bertrand to glance a header past Cech.
Myhill made a fine save to turn aside Sturridge's flick and then clung on to Hazard's shot.