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The Latics have lost all three of their games this season but could have picked up a point against Walsall were it not for Erhun Oztumer's 90th-minute winner.
Sheridan's side have conceded seven goals in their opening three fixtures.
"I'm watching players who I'm telling at half-time are excellent, and in the second half they're not at the races," he told BBC Radio Manchester.
"I pick the team, I need five or six players. I need changes to the system and some players, unfortunately, are not good enough at this moment to get in the team.
"A couple of them are playing as well and I don't like saying that, but we're not going to go where I want us to go."
Saturday's defeat by Walsall means Oldham are now without a win in their past six games in all competitions.
"Luckily for me only two games have gone in the season, and I want to sort it out as quickly as I can," Sheridan continued.
"There's lots of positives to take from [the defeat by Walsall] but we've only had a drink and sat down for 10 minutes, so why am I watching a different team in the second half?"
Oldham boss John Sheridan says he needs at least five or six new signings before the transfer window shuts.