Manchester United sent a young Mainoo to the Owls to work with the senior squad.
Windass recalled to Pitch Side: "He came to Sheffield Wednesday two seasons ago for like three weeks training to get experience training with men. He trained with us every day for like three weeks. He was serious.
"Man United sent him to us to get experience training with the first team. He was good. We didn't know who he was at first. We were laughing because we thought he was on trial.
"I was like: 'He's all right'. Normally you get trialists who are not very good. I was like: 'He's a decent little player, him'. He wasn't there three weeks later and I was like: 'Where's that trialist gone?'
"They sent players to us. They didn't come to learn, but they were getting smashed every day by League One players. He was training with us for about two or three weeks. He was good. I think they were thinking about doing a loan.
"I didn't even notice and then I saw him playing about two or three weeks ago and Barry Bannan text me to say he was the kid who was training with us."