Ben Tate carried the rushing load for the Texans (except for 10 carries) through the first three games. He looked exceptional, averaging over 100 yards per game. He also ran with a rare physicality, like Marion Barber or a poor man's Adrian Peterson. He also started a conversation about who the Texans would end up signing to a long-term contract if they couldn't keep both.
That debate should have ended on a play in which neither of them touched the ball. When Schaub tried to sneak the ball, and ended up injuring his foot, it became apparent that there is a big difference between a good back and a great back.
While Tate is a good back, he is not nearly the back that Foster is. Foster has a rare ability to press the hole at full speed while never overrunning his blockers, all the while being able to react to the cut-back lanes that the zone-blocking scheme naturally opens up. This rare combination of balance and tempo make Foster the best pure running back in the NFL today.
Despite how amazing Foster is carrying the ball, the biggest disparity between Foster and Tate is in the passing game. In the eight games that Foster has played in (including the Miami game that he only played a portion of) he has three 100-yard receiving games.
When Andre Johnson is not in the game, Foster is the best receiving threat on the team. That is not to say that he is the best receiver outside of Johnson (both Owen Daniels and Kevin Walter have better hands and are better route runners) but he is the most explosive and dynamic weapon that Matt Leinart will have as he prepares to debut.
Leinart should rest easy knowing that not only will the defense have to respect the run on every play but, when the Texans do ask him to pass, he will have arguably the best check-down option in the game today.
The Texans probably won't put the ball in Leinart's hands often (Schaub had only 16 attempts against Tampa Bay), which is great news for Leinart, as well as the tandem of running backs the Texans have. Foster is human and will need a break at some point during the upcoming games, which is why the Texans are fortunate to have two starting-caliber running backs on their roster.