Di Matteo’s men struggled to contain West Brom’s dynamic duo Shane Long and Peter Odemwingie, with the pair both scoring past Petr Cech at the Hawthorns.
Chelsea, who left Ashley Cole and Branislav Ivanović on the bench on Saturday, have now picked up just two points from their last four games, and Di Matteo admitted his side must improve defensively.
“Clean sheets certainly give you a chance to win games and we certainly have to improve on that side,” Di Matteo told Sky Sports.
“Defensively we just weren’t good enough. Of course we miss John [Terry]. He is an important player for us.
“He gives us a lot of confidence at the back and in general to the team.
“It’s a shame that he is injured – but we will have to do without him until he comes back – and improve.”
Di Matteo added: “It’s trying to find the right balance for us, whereby we can still create chances to win games.
“Maybe it’s a time where we might have to change a bit in terms of our selections and the way we play but certainly we have to start with paying more attention defensively.
“Against West Brom, the first goal came from a cross and our defenders should have done better in dealing with that cross.
“We also had enough bodies in there to deal with the second goal as well.”