Do knew

"Lingard made a great run behind the defence, came back inside Morgan and hit a shot that was kneed into the net by Fellaini. I don’t know how much Fellaini knew about it. I do knew he was in an offside position, just, so Leicester were hard done by there. It’s been a good day for Jose Mourinho though: two of his substitutes scored, and the other made the second goal." (The Guardian's live sport report.)

Is "I do knew" correct in that comment above?