Paul Sturrock warned Southend they must play better than they did in their 2-0 win against Plymouth if they are to beat the top sides.
The Shrimpers saw off crisis club Argyle thanks to a Liam Dickinson penalty and a header from Peter Gilbert in the second half of Saturday's League Two clash at Roots Hall.
But Sturrock, who had two spells in charge of the Pilgrims, said: "In the first half we did not perform very well and did not get started.
"I think Tuesday night (when 10-man Southend beat Gillingham 1-0) took a lot more out of us than we thought. We looked leggy and Plymouth did well.
"We gave them a bit of a rocket at half-time and changed one or two things around and have got the win.
"But we are going to need to play better than we did in the first half if we are to beat the more dangerous sides that will come here."