The Hammers lost 2-0 on Saturday with Dino Mavropanos also seeing red.
“We have to do better,” said Souček.
“Their first goal was tough to take. I think both teams could’ve done better in the first half, and we’d discussed things going into the second period.
“We knew that first goal could be really important, so it was really frustrating when they scored so quickly after half-time.
“We were full of positive emotions after the win the other night, and today is quite the opposite. But we’ll keep working, and hopefully we can come back stronger.”
Soucek added, "The message has been clear, about how we can’t change everything straight away, and that it is a process. But, we still have to be motivated for the challenge, and to have the mentality to fight for every point in every game.
“The fans have always been amazing, and I have such good memories of when they were travelling all over Europe to support us. That’s where we want to get back to, and we have to fight to do that.
“We have some time in training now, and I think we’ll see during the week what the manager’s main principles are. We need to keep going, and keep practicing, and I’m hopeful we’ll get there.”