It's a story that's been told over and over, with different characters and sometimes different outcomes. A player whom we've been told (and witnessed) to be an important fixture in a team's rotation ends up departing before year's end (either via trade or free agency), and the team is left to try to fill in the hole. Sometimes it's with one player, and sometimes it's by committee, but in either case the hope is that the replacement(s) can provide a reasonable facsimile of what the departing player did, or even a completely different set of attributes.
We saw this last season, as Oklahoma City attempted to replace James Harden's super-sub role with Kevin Martin. Martin could not provide the ballhandling/playmaking facet of Harden's game, but scored roughly the same amount of points (18.4 versus 19.3 points per 36 minutes), and was a superior 3-point shooter and off-ball offensive threat (moving off screens). As such, the Thunder was able to make do with Martin to the tune of an Oklahoma City franchise record of 60 wins.
Here are some high-profile situations around the league in which teams had to find a way to replace what was lost.
Though Howard's tenure in Los Angeles generally was looked upon as a failure, he still was an incredibly productive player for the Lakers, leading the league in rebounds per game (12.4) and ranking among the NBA's best in points per possession in pick-and-roll situations (1.3). And while Lakers fans might have said "good riddance" when Howard left for Houston, the fact remained that his presence needed to be filled on both ends of the floor.