Rooney is expected to sign for DC United this week.
“It's great to have had the opportunity to work with him," said England No1 Pickford.
“He was a great lad around the place and has got unbelievable talent. I was growing up watching Rooney as an England legend and as one of the top players in England in my lifetime.
“That's why he's the top goalscorer for England. But I don't know what's going on at the club, I'm just focused really on getting as far as we can in the World Cup for England."