Fletcher scored 11 Premier League goals in his first season at the Stadium of LIght, but has failed to hit the same heights in 2013-14, finding the net only three times.
The Scot has been hampered by an Achilles injury, but his poor form has been a worry for a side that sits second bottom of the top flight.
Sunderland return to league action against Crystal Palace on Saturday after a League Cup final loss to Manchester City and a FA Cup quarter-final defeat at Hull City.
Fletcher started his first game since January in the reverse against Hull, and Poyet believes the former Wolves man will be important as the Wearside club aim to retain their Premier League status.
"We need him. A lot," Poyet told the Shields Gazette. "I'm pleased that he played for Scotland (against Poland) last week.
"I know managers are always afraid of players going away on international duty, and I understand why.
"But you cannot plan when players are going to get injured. Playing for your national team is a great experience, and you want to win."
Fabio Borini has played as a lone frontman for much of the campaign for Sunderland, but was left on the bench at the KC Stadium.
The Italian, on loan from Liverpool, is likely return against Palace, but Poyet did not rule out the possibility of playing with two strikers.
"There's going to be games when it is right to do that (play with two strikers). But it will depend on the game," he added.
"We know what Fabio gives us, it's clear, and depending on the game, we will play one way or the other."
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