Harry Lennon says Charlton must get back to winning ways or risk missing out on the play-offs.
The Addicks dropped out of the top six for the first time in three months after losing to Southend on Boxing Day.
Charlton are now two points behind sixth-placed Peterborough.
And Lennon, who is pushing for a start at league leaders Wigan on Friday, said: “Everyone is still positive.
“We’ve got a good team spirit, but we've got to work harder.
“There are key players missing and with those people missing we've got to work even harder than we have been. We probably haven't in the last few games.
“So we’ve got to get back at it sooner or later otherwise we’re going to be way out of the play-offs and it’s going to be too late before we know it.”
Harry Lennon | Picture: Benjamin Peters Photography
Lennon replaced Ahmed Kashi at half-time in the Southend defeat to make his first league appearance in 14 months.
A serious groin injury stalled his career after impressing in Charlton’s 2015/16 season, which ended in relegation.
The defender, 23, added: “In the relegation season I played quite a few games and I felt, on a personal level, I was doing well playing in the Championship every week.
“But last season it was a tough year and I got the injury and kept trying to come back, it wasn't quite there at all.
“Now, hopefully I can get back to that level, push on, start again and get a run of games.”
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