The Germany forward could have stayed at Leipzig until their European run in August is complete.
Werner will be hoping to hit the ground running when he links up with Chelsea, with Di Matteo confident that a prolific marksman can make a positive impression in the Premier League.
Leipzig's season, however, is not finished with the club having progressed to the quarter-finals of the Champions League, though Werner has made a decision to play no part in August's latter stages as he prepares ahead of the new Premier League season.
The entire Chelsea-London package was the best for me, my game and my career.
"Of course, it is hard to give up the kind of oasis of wellbeing I had in Leipzig for years, but you can only get ahead in a career if you take the next step".
"It was clear that all parties - Chelsea, RB and my side - should and must settle this".
He was considered to have outgrown his surroundings at RB Leipzig - with his time at the Bundesliga outfit ended in style of the final day of the 2019-20 campaign as he became the club's all-time leading scorer.
Werner admitted the risk of injury was amongst a host of reasons for opting out of Leipzig's Champions League squad, but says he will be a keen supporter of the club's progress despite his departure to English football.
A clamour to secure his signature was set into motion, with many aware of the benefits that the proven 24-year-old could add to their ranks.
"I am forever connected to Leipzig and RB, will sit in front of the TV during the quarter-finals and keep my fingers crossed - maybe I can even be on site in Lisbon", Werner said.