Ahead of United's Europa League quarter-final first-leg against Lyon, Onana suggested that the Ligue 1 side are a “very good team” but that “I think we are better than them” despite his side being 13th in the Premier League.
Matic, who is now 36, did not take his comments lightly and hit back on Wednesday afternoon in a shocking statement that will only add fuel to the fire ahead of the clash.
“To say that, you need to have cover to say something like that. Onana, I respect everyone, but to say that, then you need to give an answer.
“If you are one of the worst goalkeepers in Man United history, you need to take care what you are talking about.
“If David de Gea, or Peter Schmeichel, or (Edwin) Van der Sar said that, then I will question myself.
“But if you are one of the statistically worst goalkeepers in Man United modern history, he needs to show that (they're better) before he says.”
Onana took to social media right after Matic’s comments and took a dig at the veteran midfielder, who never won a trophy with the Red Devils during his time there.
"I would never be disrespectful to another club.
"We know that (Thursday) will be a difficult game against a strong opponent. We focus on preparing a performance to make our fans proud.
"At least I've lifted trophies with the greatest club in the world. Some can't say the same."