The Hammers headed into the international break on the back of their first defeat of the new season, losing 1-0 to Stoke City in the Premier League.
Speaking at a press conference, the manager said: "It has been a long two weeks. I think since I've been here it's the first game I've ever lost before an international break, if memory serves. You always want to go into an international break with a positive result.
"And then the trials and tribulations of the window shutting after that and then, of course, earlier this week, the setback with Andy Carroll. So it has made the last two weeks very difficult for me personally.
"But the squad is still very focused and, I hope, determined to deliver the performance that we know we can produce at Southampton on Sunday. All our international players are back free from injury, so we look forward to the game and I'm looking forward to a positive performance from the players."
West Ham are eighth in the league table after the first three games.