The London club head into the international break six points clear of second-placed Manchester City with a game in hand after Loic Remy came off the bench to score the winner with his first touch 13 minutes from time.
Chelsea were seemingly coasting to victory when impressive finishes from Eden Hazard and Diego Costa put them two goals up after only nine minutes at the KC Stadium on Sunday.
He said: "The title race should be over, in normal conditions Chelsea should have eight, 10 or 12 more points than we have but football is unpredictable.
"The reality is that we have a six-point lead, the best situation we've had all season. The maximum distance we had was eight points but at that point we had 20 matches to go.
"At this moment we have a six-point lead with one match in hand, which could help us or not. I'm pretty confident, I believe in my players, I believe we can do it but I know it's difficult."
He revealed: "It's a hamstring injury. When a striker is playing, the team needs a goal to win the game and he says it's over for him, it's over.
"He tried to play the Champions League final with Atletico [Madrid] and was injured, he has that fragility, we know his hamstring is not a strong one.
"He works hard in the week to compensate for the weakness he has there but the injury can come. We have Remy and [Didier] Drogba, we never cry about injured players so let's move on."
