West Ham could move into the top four with a win against Sheffield United tonight.
Soucek is their top scorer this season with eight goals and Coufal insists the midfielder can still improve.
“He is incredible,” Coufal said in an interview in tonight’s match programme.
“I have known him for a long time, so I know he can still get better and better because his physical attributes are unbelievable.
“I have to say it’s hard to defend against a player like him and I hope he will stay here for a long time.
“Soucek is my best friend in the Czech Republic so he has been very, very helpful to me since I arrived at West Ham, and I’m very grateful to him.”
Coufal says West Ham are targeting a top-six finish.
“When we finish the season, if we are still in sixth position, I’ll meet all my teammates and we’ll go out for a good party,” he said.
“It will be crazy and unbelievable if we finished the season in sixth place, but everybody at the club is doing everything for it.”
West Ham supporters can order their programme for the visit of Sheffield United here.