Victor Moses' header snatched all three points for the Champions League holders after two strikes from Willian had twice equalised efforts from Fernando Torres and Oscar
Willian's double had brought the Ukrainian side back into the game after Fernando Torres' fortunate effort and Oscar's incredible 45-yard strike but the Italian was understated when asked exactly what the Nigerian's late effort meant for the European champions.
"It means we get three points," Di Matteo told Sky Sports. "[It was] crucial for us to win this home game; it was always going to be a tough game to beat a side with little details.
"They showed again how good they are. It's probably going to go down to the last game, the last kick of it as well.
"The spirit the group showed again to just keep going to the end. We knew we probably needed to score a goal from a set-play at the end and we did so. It was good that Victor got on the end of the corner."
The Blues were made to sweat by Shakhtar, whose Brazilian stars shone at Stamford Bridge, but Di Matteo was satisfied with his team's efforts in defence as well as attack, adding: "In the first half we put them under pressure and that's how we scored the first goal and then we managed to go ahead again with good pressure.
"In the second half, after they equalised, we tried but they are a good side but probably didn't have as many chances as normal.
"We knew what kind of pressure we were under and it was a must-win game for us and you didn't want to expose yourself too much.
"We did a little bit because we wanted to win the game but we knew that we could have scored a goal with the last kick and that's exactly how it happened."
Oscar hit another stunning Champions League goal at the Bridge, to follow his equally impressive goal against Juventus, cutting out Andriy Pyatov's poor clearance and firing in a half-volley 45-yard effort, but his manager was not surprised by the stunning nature of the strike.
"[He showed] tremendous ability because it wasn't easy at all, he got the ball down on the half-volley, it was very difficult but he's a Brazilian international," Di Matteo remarked.
John Terry was only named on the substitutes' bench, with the Blues boss revealing before the game that the captain was lacking match fitness, but the 42-year-old insisted that his skipper will be back in the starting XI soon.
"He's a massive player for us," said the Italian. "We are glad that he's back available for us and he's going to be our leader going forward."