The World Cup rather scuttled Chelsea's pre-season. Brazilians, Belgians, Nigerians and the lone German were allowed time to recuperate; as such, Jose Mourinho has not been afforded time to let new parts integrate with older ones.
Chelsea play host to Real Sociedad on 12 August; the Stamford Bridge friendly will likely represent Mourinho's best opportunity to play his first-choice XI for 90 minutes. With the Blues visiting Burnley's Turf Moor on 18 August to kick off their Premier League campaign, that does not leave much time for cohesion.
Thus far into the pre-season, Chelsea have not played great—more like "serviceably." The gradual addition of World Cup stars proves to be the next gear, however, the system is still being tinkered and formatted.
Mourinho has started with the 4-2-3-1 in most games, but with Chelsea trailing three of five games, he was forced to take a more aggressive tack—changing to a 4-3-3 and even playing Branislav Ivanovic as a third striker.
Once the Portuguese has his full complement of stars on hand, going with a 4-3-3—given the personnel—is sure to elicit the best Chelsea performances; hence, this "best XI" is based on that assumption.