Alan Curbishley has told Carlos Tevez to impress in training to win his place back in the West Ham starting line-up.
Tevez was dropped to the bench for Curbishley's first match as Hammers boss, with victory against Manchester United giving them renewed hope in their fight for Premiership survival.
The Argentina forward was brought to the club just before the transfer window closed for the winter but his arrival, along with compatriot Javier Mascherano, has been blamed for having an unsettling effect on the squad.
The Hammers are still recovering from eight consecutive defeats just after the pair signed from Brazilian side Corinthians, a sequence that played a part in Alan Pardew's dismissal and Curbishley's appointment last week.
Curbishley, a former Hammers player, picked his side for the United victory based on two brief training sessions and, despite Tevez showing improvement in recent weeks, dropped the Argentina international to the bench following defeat at Bolton the previous week.
'He took it well,' Curbishley said. 'He understood and I've said to him that the players who didn't play will be in training the next day and will show me something, because I need to see them perform.
'With the Christmas period they are going to have to show me in training or when they get their chance because there are no reserve games.'
He added: 'It's a different situation. A new man has come in, they've got to express themselves in training and when they play, and we take it from there.'
Mascherano was not on the bench against United and Curbishley will now assess his squad ahead of next week's clash against Fulham, as well as getting to know his players' names after a whirlwind week at Upton Park.
'We came in Friday morning and trained for an hour, we were in again on Saturday for 15 minutes so I didn't know what the side was until the morning of the match,' Curbishley added.
'I was also wary of plying them with too much information - I'm just getting to grips with their names let alone anything else.'
Curbishley will have the opportunity to bolster his squad when the transfer window opens next month, and he will also have Shaun Newton available although he may have to wait until February for Dean Ashton to complete his recovery from a broken ankle.
What Curbishley, however, wants to put into his squad above anything is the mentality to fight their way out of the bottom three in the Premiership.
'The trouble with West Ham is that they haven't been down there before,' said Curbishley. 'It's a young side that was on the crest of a wave last season and they probably haven't lost four or five in a row before.
'This year they have and perhaps they didn't know what was going on. I've been there before so I hope my experience will cope.'
Victory, courtesy of Nigel Reo-Coker's winner, also meant United's lead at the summit was cut to two points following Chelsea's win at Everton.
United boss Sir Alex Ferguson has called for his side to be sharper in front of goal when they play away from Old Trafford.
Ferguson said: 'We've been speaking about it for a few days, we've been throwing a lot of chances away, away from home, and it was another example of that.
'We played some fantastic football and had some excellent opportunities but our finishing should have been better.'