But the Senegalese is aiming to bounce back and recapture his best form in the closing weeks of the Championship campaign.
The 29-year-old, in a revealing interview before the clash with his former club Wigan Athletic at St James's Park tomorrow, said: "This season has been the poorest season of my career.
"It’s tough mentally.
"Sometimes you lose a bit of confidence and you lose confidence about how you can play.
"It’s not easy, but this is football, and it happens sometimes. You just need to keep faith in yourself."
Speaking in United's matchday programme, Diame thinks he can take inspiration from one of teammate Matz Sels international colleagues as he bids to get back to his performances of old.
"I can give an example – Eden Hazard," he said.
"He had a difficult season last season, but he knows he’s a massive player, a good player. This year, he’s back again."
Diame was brought to these shores by then Latics boss Roberto Martinez from Spanish outfit Rayo Vallecano back in 2009.
He then spent three years at the DW Stadium, before heading on to West Ham United, then Hull City.
Reflecting back on his debut in English football he admits he was nervous about switching from the continent - with the language barrier a particular worry.
But he says his time in the north west is a period of his career he will never forget.
"I remember that it wasn’t easy at the beginning," he said.
"To be honest, I wanted to stay in Spain before I came, and it wasn’t an easy decision.
"I didn’t speak English, and people were telling me that in the Premier League the contact, the physicality, is difficult, and the players are stronger than in Spain.
"It wasn’t easy, but when I arrived in Wigan, I found Roberto Martinez – who is Spanish of course – and the Spanish football he liked to play, and I enjoyed it.
"I had three years there, and it was amazing. We always fought relegation and to stay in the Premier League, but it was good to play with Wigan.
"I will always remember this club as a good one in my career."