The 26-year old departed Aberdeen in 2010 to return to Parkhead after leaving in 2006 and admits he came very close to joining the Blades.
"I met the manager Kevin Blackwell and, after they had finished eighth in the Championship the season before, they fancied their chances of going up.
"I near enough agreed terms. It was a good opportunity, but there was only one option when Celtic then came.
"I don't think the United manager was very happy about it. I hadn't quite given my word but it was getting to that stage.
"It's one of those things. Celtic was the best decision for me."