The Baggies were embarrassingly trounced 6-0 in their first Premier League game at the Champions.
Di Matteo said: "I think it was a slap in the face that said: 'hello guys, you're in the Premier League now, this is the top team and you need to learn very fast how the trade is plied here'.
"Preferably I could have done without it but I think the players have been magnificent to bounce back from that defeat - especially with the way we did it because we played really well against Sunderland.
"What I like about this team is that they have always shown a reaction.
"They have always bounced back from a bad result and I think they have shown the team spirit that we have here and the personalities that we have here to be able to pull everybody in the right direction and get the right performance.
"You never know what the result is going to be but you need the right performance and I am delighted with the progress that we have made since the beginning and that game at Chelsea."