Chelsea should not relax on the transfer window, Thomas Tuchel will need to look at strengthening the offense, Chelsea attack is poor, they are not scoring goals. Sterling is supposed to be Lukaku's replacement, Lukaku didn't have a great season under Thomas Tuchel but he was still Chelsea's highest goal scorer. Chelsea need goal scorers, their defense is strong but they need goals yo win matches.
I don't think Chelsea's defense can be considered strong, and i also agree with you that they prolly need a striker (Werner is poor), i wouldn't consider Raheem Sterling to be a striker, he doesn't have that confidence in front of goal and he misses really clean chances to put the ball in the back of the net, i know he scored a plethora of goals for City, but that was because of how the team played, City makes everything so easy, if Sterling is to play as a striker in Chelsea, where the service behind him isn't as slick as at City, he would struggle to score goals.
Having said that, they also need to sign another central defender, Koulibaly isn't enough in my opinion. I do not think Chelsea will be fully ready when the season starts, even Tuchel doesn't think so too, he said the same thing a couple of days ago; before they prolly start to find their feet, it is possible that either City or Liverpool might have started running away with the title.