The center half has been a vital member of the team under boss Chris Wilder, as he was under Paul Heckingbottom.
However, the club have yet to make him a big offer to tie him down for the next few years.
Advertisement
Advertisement
It's part of the game," he told the Independent.
You could be somewhere for six months, you could be somewhere for six years. You just never know."
It's the longest I've spent at a club in England and we've had a lot of success," he added.
When I look back on my career so far, all my best moments were with United. We got promoted twice to the Premier League, had a great year the first year, I broke into the Ireland team while I was at United.
I've loved every minute of it. It's a fantastic club. The Premier League is where the club deserve to be."