Roberts spent the first half of the campaign on loan at Middlesbrough from parent club Manchester City but made just four league starts for the Teessiders after returning to the club in October.
Boro were happy to keep the playmaker for the remainder of the season and Warnock challenged Roberts to stay at the Riverside and fight for his place.
Yet Derby’s interest gave the 23-year-old a chance to go and gain more regular game time at a Championship club.
“I said to Patrick he’ll do well at Derby because they play a different game,” Warnock told the Mail when asked about Roberts. “They’ll get it into his feet and he can run at people.
“With us, as I said to him, I signed him in good faith, I thought when I signed him he could have a big part to play.
“But he couldn’t play wide in our system because he couldn’t track full-backs and when I played him in the hole, if we played against a good three in there which most teams seem to have, we were outnumbered.
“It just didn’t seem to work out for him whereas I think he’ll do well at Derby, the way they play he’ll have more opportunities.”
The arrivals of Yannick Bolasie, Nathaniel Mendez-Laing and Neeskens Kebano at Boro also played a part in the club’s decision to allow Roberts to leave.