One in, one out at Dons
The 19-year-old has penned a two-year deal at The National Hockey Stadium, after being told he could leave Pride Park.
"Izale is at a stage in his career where he needs to be playing regular first-team football," said McLeod.
"We thank him for his service to Derby County and wish him well with his new club."
Darlington has signed on at Vicarage Road after agreeing personal terms and passing his medical.
"When I first came here everything was fine, but I woke up one day and my groin was a little tight, but I'm glad the scans came back giving me the all-clear," Darlington told the club's official website"I had a few clubs interested in me but Watford was always my No. 1 choice,'' he added. "I spoke to Ray and once I knew he wanted me I was more than happy to come here.
"I obviously know Terry Burton from my Wimbledon days, so I know we will try and play good football and that influenced my decision.''