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.
"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."
"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."
