Ecuador coach Reinaldo Rueda will have come away from Saturday's 3-1 defeat against Mexico in Arlington, TX with much to ponder. There were some positives, but also a number of negatives that will need to be ironed out prior to Ecuador's World Cup opener against Switzerland on June 15. These are the main takeaways from the match. Formation works out OK Rueda sent out his side in a 4-1-4-1 formation with Segundo Castillo holding in front of the defence and Jaime Ayoví, normally a striker, lining up alongside Edison Méndez in central midfield. Fidel Martínez started up front, with both Antonio and Enner Valencia on the bench. Ecuador began the match with good energy, pressing Mexico high up the pitch and forcing a number of errors. They were compact and well-organised, holding their shape until opportunities presented themselves to initiate a press. Without a strong central presence up front it made sense to play in this manner as when the ball was won, there was plenty of support close to the man in possession. Jefferson Montero and João Rojas got into some good crossing positions but were unable to produce a quality final ball into the area. Martinez, while hard-working out of possession, wasted a couple of decent break opportunities with lazy attempted flicks. Mexico gradually worked their way into the match, but Ecuador defended well, with the midfield holding station and central defenders Frickson Erazo and Jorge Guagua proactive in stepping forward to intercept. As against Netherlands, Ecuador were well drilled positionally and it took a moment of real quality to break them down: Oribe Peralta neatly teeing up Luis Montes for a superb curled finish into the corner of the net. The experiment was ended when Castillo was taken off injured near the end of the first half and Ecuador returned to their normal 4-4-2 formation after the break. Segundo Castillo's injury Castillo's participation in the World Cup is in serious doubt as a result of an injury he picked up in a sickening clash with Mexico's Luis Montes in the 35th minute. The Mexico midfielder came off worse, suffering what looked to be a fractured tibia, but Castillo was also stretched off. Early reports suggest he has suffered a torn ligament, but the full extent of the damage will only be known after he undergoes an MRI scan. If Castillo is ruled out of the World Cup it will be a major blow to Rueda, who was counting on the experienced midfielder to form a solid central base alongside Christian Noboa. In his absence, Carlos Gruezo is the most obvious like-for-like replacement but his youth may count against him. Indeed, Rueda could turn to Oswaldo Minda, who was part of his preliminary 30-man squad but did not travel to the United States for Saturday's match. Enner Valencia adds incision in attack Rueda gradually introduced his first-choice forwards in the second half, with Enner Valencia coming on for Jaime Ayoví at half-time and Felipe Caicedo replacing Martínez with just under 20 minutes left to play. The pair dovetailed well in their short time together on the pitch, with Valencia's powerful acceleration causing problems for the Mexican defence. The neat but not particularly incisive attacking play of the first half gave way to a more direct approach. Valencia scored Ecuador's only goal of the match when his low free-kick from the edge of the area was deflected into the back of the net. He also had their best other chance of the second half, racing in behind the defence only to be foiled by Ochoa as he tried to round the Mexico goalkeeper. He will now hope for a chance to impress from the start against England on Wednesday. The defence remains error-prone Ecuador's defending became more ragged in the second half, with Erazo and Guagua both snatching at clearances in the first quarter hour after the break. The switch to a 4-4-2 formation removed some of their protection and suddenly the problems evident against Netherlands re-merged. Mexico's second goal was as brilliantly taken as the first, with Marco Fabian firing home a bouncing ball from long-range, but the third was all Ecuador's doing. Guagua cleared a poor cross straight to Giovani dos Santos, whose shot bounced off the post and back off goalkeeper Máximo Banguera into the back of the net. Rueda's team have looked well-organised in both of their pre-World Cup friendlies to date, but individual errors continue to undo the excellent preparatory work of him and his staff.