For Dempsey it's the ability to provide a versatile scoring option from midfield with a combative personality and a real touch of flair to his game. Dempsey can act as a capable forward as well, though I think it has become clear by now that his real value comes as a midfielder with an eye for goal and aerial presence.
For Zamora it is excellent hold-up play with setup and foot skills atypical for the archetype of forward he plays as. Many forwards can win the ball and hold it up for an oncoming teammate, but few can control the ball and play it through like Zamora.
One of the major reasons Roy Hodgson's revitalization of Fulham was so successful was his acquisition of Zamora, a player whom both Mark Hughes and Martin Jol heaped effusive praise upon when arriving at the club. When Zamora was having a terrible time finding the back of the net during his first campaign with Fulham, Hodgson defended Zamora vigorously, boasting about how refined Zamora's other skills were. It may not have showed in his goal totals, but it did in the much more overlooked assist category.
Zamora has lead Fulham in assists in each of the last three seasons, with the demonic totals of six, six and six (he tied Danny Murphy for team lead in 2008/09). Even more impressively, Zamora recorded his team-leading assist total last season while only appearing in 19 matches during an injury shortened campaign. Over those same seasons, Dempsey's assist totals were two, three and three.
With Zamora out of the lineup, Fulham also lack a target option capable of taking on the toughest of defenders. Dempsey seems to fit the billing, and really provides the only logical replacement option on Fulham's roster. The American boasts a size advantage over Zamora and does well in the air attacking balls on set pieces, but lacks in Zamora's much valued set up role.
The first two games of Fulham's Premiership campaign have served as a reminder of just how important Zamora is as a target for Fulham. Using the Guardian's Opta-powered interactive chalkboards, I can hopefully show this convincingly.
Schwarzer completed 25 of 36 passes during the match. His passing rate was relatively high because of how frequently Fulham built up play from the back. But on Schwarzer's passes that went to at least the halfway line or further, he completed six of 14 attempts. Of those six, four were controlled by Zamora in 74 minutes of play, one by Dempsey, and another by Damien Duff. Andy Johnson was unsurprisingly shut out.
While the match ended in a scoreless draw, few would deny that Fulham looked the superior side and the more likely of the two to win.
Now, Schwarzer's chart against Wolves:
A lot uglier and considerably redder, reminiscent of a brutally graded test and one that Fulham obviously failed. Schwarzer only managed to complete 17 of his 44 passing attempts against Wolverhampton. Of the 28 balls he heaved to or beyond the halfway line, he only managed to complete four. Two were won by Dempsey and one apiece by Pajtim Kasami and Moussa Dembele. Of note is that Dempsey and Kasami each spent time in midfield and forward roles, with Dempsey starting the match slightly withdrawn behind Johnson before dropping back into midfield about midway through the first half.
Wolverhampton's Wayne Hennessey, meanwhile, heaved the ball far forward with regularity and enjoyed the luxury of his teammates playing effectively in the air:
Hennssey attempted 50 passes, completing 29. Of those passes, he kicked 39 to or beyond the halfway line and his teammates won the ball 18 times.
This provides a tremendous territorial advantage.
Not only did Wolverhampton typically kick the ball forward a farther distance than Fulham, they won it much more frequently as well, succeeding 46 percent of the time versus 14 percent for the Cottagers. Knowing Wolves' generally bludgeoning style of play, this type of strategy in distribution is unsurprising. Because they were able to win the ball in such advanced positions so regularly, they were able to bypass possession-oriented build up play that they are unequipped to excel at.
Fulham without Zamora was left with only the option to build up from the back and seemed unable to handle how hard and fast Wolves came at them.
With Zamora in the lineup, spearheading the attack, opposing defenses must respect his ability to win the ball in the air and play it off to Johnson, who unsurprisingly looks considerably better with Zamora in the lineup than without, or earn fouls against over-matched defenders.
While Dempsey is more than capable of winning fouls and the occasional duel in the air, he doesn't flourish in target play like Zamora does. Dempsey wins fouls taking on defenders and, admittedly, going to ground easily. Zamora tends to simply get mugged.
You can find games where Dempsey is dominant in the air as a forward, particularly in Fulham's early season 2-0 win over Wigan last year in which Dempsey scored a brace. He is, however, more hit or miss at this type of game and is a much more selfish player than Zamora. Once again, this is neither good nor bad, it just is. If Dempsey weren't so selfish, he wouldn't score as many goals. If Zamora were more selfish, he wouldn't link play up as well as he does.
Dempsey is Fulham's second best target option. And while he may be better than most in the air and posting up, Zamora is in a league of his own. Without him, Fulham are a much frailer side. It showed on Sunday.