Arsenal's 5-3 win over Chelsea on Saturday at Stamford year Blues boss Andre Villas-Boas.
Chelsea conceded five goals at home in a match for the first time since 1989, while the team has now gone nine straight games without a clean sheet in the league, its longest stretch since 2003.
An air-tight defense was one of the hallmarks of Chelsea's team under Jose Mourinho as he led the club to successive league titles in 2005 and 2006.
But since his departure, the Blues have gradually gone away from Mourinho's more conservative defensive approach in favor of a style of play that is pleasing to the eye but hasn't always yielded the desired results.
The Chelsea defense was ripped apart by Arsenal on Saturday, but Villas-Boas was quick to defend his team as well as his tactics.
"Before this game we were [one of] the three best defenses in the Premiership so I would argue with anyone that our defensive organization was a problem before this game," Villas-Boas said. "We committed mistakes today that we will try to evolve."
But the loss to Arsenal was more than just an unlucky break. It brought to light a few real concerns that Chelsea must address going forward.
Terry has been an ever-present for Chelsea during its best times in recent years, but he has never been the quickest player. And when paired in the middle with the lumbering Branislav Ivanovic, the duo severely lacks mobility, something Arsenal exposed on a few occasions.
Right back Jose Bosingwa seemed more interested in playing right wing on Saturday and left Arsenal with acres of space down the flank, while the fact that Villas-Boas has been employing a three-man midfield also leaves his defenders more susceptible than they used to be.
Chelsea's three-pronged attack on Saturday of Juan Mata, Fernando Torres and Daniel Sturridge will give any opposing defense trouble. But the midfield of Frank Lampard, John Obi Mikel and Ramires hardly offers the back four much cover.
Against most teams in the Premiership, Chelsea's defense has held up fine because the club has the kind of offensive firepower to cancel out the odd goal it gives up.
But against United and Arsenal, the two best attacking teams that Chelsea has faced this season, the results haven't been good.
And maybe it shouldn't come as a huge surprise considering the recent transfer activity of the club.
Instead of spending some money to upgrade an aging back line in need of reinforcements, owner Roman Abramovich has opted to hand millions of dollars to Fernando Torres and Juan Mata, further bolstering an already strong attack.
Unless something changes philosophically at the club, Chelsea fans will be treated to plenty of goals and some eye-catching offensive stuff.
But like Saturday, they may also find their team on the wrong end of an entertaining result.