Southend player-coach Graham Coughlan has accused some of his team-mates of overestimating their own ability.
Saturday's 3-0 loss at Cheltenham was the Shrimpers' first league defeat since 10 September and ended a six-week run at the top of League Two.
"There's a couple of our players that think they're better than they actually are," Coughlan told BBC Essex.
"They may fall by the wayside over the next couple of years. They may fall out of the game."
Despite their impressive form in the league, Southend have started to show signs of a dip in form, having suffered a Johnstone's Paint Trophy defeat against Swindon last week and failing to win any of their last four games in all competitions.
"They need to realise that without hard work, grit and determination, you won't get far in this game," added 37-year-old Coughlan, who joined boss Paul Sturrock's coaching staff in the summer.
"I've known Paul Sturrock long enough to know that if you fail to display passion, belief, pride in the jersey, you don't spend long under his management."
The Roots Hall side's future in the FA Cup will be decided on Tuesday when they travel to Oldham for a second-round replay and Coughlan expects a positive response from the Southend squad.
"Tuesday is a massive game now," he said. "Not just for the team but for individuals.
"There's questions being asked and fingers being pointed. So it's up to individuals to react and answer those questions.
"You don't become bad players over night. But you have to work hard and dig deep."