Martin O'Neill is concerned about next weekend's international break disturbing Sunderland's "rhythm".
The announcement of international squads this week saw a number of Black Cats players called up for duty, including Scotland's Steven Fletcher.
"It's a big concern for me and for other managers in my position," O'Neill told BBC Newcastle.
"There's a pattern at club level and then they go on international duty and it can upset the rhythm."
He added: "You're never sure what sort of training they're doing and how they're settling, but thankfully this is the last international break for some considerable time."
Meanwhile, the Northern Irishman wants Sunderland to gain confidence from last season's unbeaten record against Manchester City when the two clubs meet on Saturday.
O'Neill's side took four points from six against City during the last campaign, with the 1-0 win on Wearside one of only five defeats for Roberto Mancini's champions.
"We were the only team to take some points off them [at the Etihad] in the Premier League," O'Neill continued.
"The players should draw some sort of confidence from that."
The Black Cats drew 3-3 on their last visit to the Etihad Stadium in what O'Neill described as "probably our best away performance".