Thierry Henry's substitution played a massive part in helping Chelsea to equalise. When the Frenchman went off with cramp in his calf it not only took something away from Arsenal but made it far easier for Claudio Ranieri's team to mount the pressure that earned them a replay.
The key is that Chelsea's back line was able to squeeze up a lot more, making the team more compact and giving them less distance to cover to mount a real threat on Arsenal's goal when they went forward.
While Henry was on the pitch Chelsea had to defend deep because of the danger he poses with his pace if he's given acres of space to run into over the top. The problem of holding a high line against Henry was shown with his goal.
Defending deep meant Chelsea were stretched as a team and had a long way to travel with the ball to trouble Arsenal when they won possession at the back. They found it hard to mount sustained pressure.
All that changed when Henry went off and Arsenal were left with Sylvain Wiltord, Robert Pires and Kolo Touré buzzing around at the front. Wiltord and Pires are world-class players, not dummies, but Chelsea found it easier to build a platform to attack from.
All of a sudden the threat of someone sprinting behind them was more or less gone. Wiltord and Pires are fast but they like to have the ball to feet and work their way through a backline with quick combination play. Even Wiltord, who's operated a lot as a central striker, doesn't like to be on a defender's shoulder and stretching people back.
With passes mostly being played in to feet, Chelsea's back four could squeeze up much closer to the halfway line and concentrate on getting challenges in. When they won the ball they could rebound it quickly into midfield and there were 20 yards less to cover before Chelsea were in or around the penalty area.
It didn't surprise me when Frank Lampard equalised because they were enjoying real pressure, and it's worth remembering another aspect about Henry's substitution. The striker's withdrawal will have given Chelsea a real lift and fresh impetus. William Gallas is lightning-quick but even he had struggled to cope with the pace of Henry.
There's no doubt Ranieri influenced the game by putting on Eidur Gudjohnsen to partner Jimmy Floyd Hasselbaink, because that left Arsenal's back line far more occupied. But I don't think Chelsea would have been able to threaten so much if Henry had been able to stay on.
