The 26-year-old has scored just three goals since the New Year, despite notching 13 in the first half of the season.
He has now gone four games in a row now without a goal, his last in the FA Cup against Arsenal.
Speaking to The Rookery End Podcast, the Super Eagles forward said international duty for Nigeria sapped him.
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“After you play at the weekend you have to fly to and play with the national team. You then play twice in six or seven days after a long flight,” Ighalo said.
“I remember that [in March] I flew from here to Nigeria. It was seven hours. Then we flew to Egypt, back again to Nigeria and then back to England.
“It was really difficult and telling on me. We are humans, not robots. So it’s really tiring.
“But what can you do? It’s your national team. We are all working to represent the national team.”
Ighalo will be hoping to help Watford finish at the top end of the league as they travel to Liverpool on Sunday, with just two games left in the Premier League.