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.
Chelsea clearly missed the physical presence at the back of injured skipper John Terry who will be side-lined for three weeks with a knee injury.
Long and Odemwingie fully capitalised on the absence of the former England defender as they punished some slack defending.
Long appeared on a one-man mission to honour his grandmother who died yesterday with another superb performance.
The Republic of Ireland international revealed a T-shirt which said 'RIP Nan' after scoring the opening goal.
Chelsea dominated possession in the early stages and Ryan Bertrand came close to putting them ahead after eight minutes.
Moses made a powerful run to the byline and squared the ball back across the danger area to Bertrand whose low left-footed drive was cleared off the line by Liam Ridgewell.
But 60 seconds later the Baggies took the lead via their first worthwhile attack.
It was the Republic of Ireland international's sixth goal of the campaign.
Chelsea tried to retaliate and John Obi Mikel's low 30-yard drive was only just off target with Myhill scrambling across his line.
Myhill then got his body behind a swerving first time attempt from Sturridge.
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.
Chelsea continued to dominate possession and Moses fired over after being teed up by Cahill.
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 Nigerian international got in front of Bertrand to glance his header past Cech for his fourth goal of the campaign.
Myhill made a fine save to turn aside Sturridge's close range flick after Mata's ball had opened up the West Brom defence.
He then went down to cling on to Hazard's deflected shot as Chelsea delivered wave after wave of attacks.
Sturridge should have made it 2-2 with three minutes left but screwed his shot across the face of goal.