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Root confirmed Denly would come into the side for the match in Antigua starting on Thursday with England trailing 1-0 after a comprehensive 381-run defeat inside four days in Barbados.
"It's disappointing for Keaton, he's done some very good things and he'll continue to keep developing," Root said. "He's very much a part of this squad moving forward but it's a great opportunity for Joe to show what he's capable of.
"At 1-0 down in the series I feel it's proactive to change things up at the top of the order."
Fast bowler Stuart Broad, controversially left out of the team for the first Test, comes into England's 12-man squad, as does spinner Jack Leach, with Adil Rashid granted permission to return home for the birth of his second child after being omitted from the final XII. Rashid claimed 0 for 117 in 26 overs during the first Test.
Pressed on whether Broad would play, Root said: "We'll wait and see, won't we?"
"Stuart adds an extra seam option and if we were to play two spinners it's a change of mentality and approach of the spinners, having a more controlled bowler like Leachy," Root said.
"It's not necessarily a reflection on how Adil has gone about things. When he comes into the side you want him to make things happen and take wickets, to show off all of his tricks. On balance we think a slightly different approach might be more effective here."
Root said Jennings' omission was by no means final.
"He is someone who is always developing, constantly improving his game," Root said. "It's been a tricky start for him over here and he'll be disappointed to miss out but he's got a very good attitude.
"I'm sure he'll go away, work on his game, keep looking to improve. I definitely don't think it's the end for him. He is doing all the right things. He felt he might not have been far away but the position we're in in this series... we're out here to win. We're not here to look forward to the Ashes."
Root was similarly open-minded about Denly's future, meaning England's search for a long-term opening pair is by no means over.
"We have to keep a pretty open mind," Root said. "He's definitely got an opportunity to show what he's capable of. He's obviously worked extremely hard, impressed on the two tours he's been on. Hopefully he can continue the fine form he had last year for Kent."
Denly, 32, returned to the international limited-overs arena in 2018 after an eight-and-a-half-year hiatus. He was also named Professional Cricketers' Association Players' Player of the Year after an outstanding season in all formats for Kent, where he scored 1729 runs and took 57 wickets.
Root believes his side is "more than capable" of bouncing back from their dismal performance in Bridgetown and, just as Denly is in a prime position to make the most of the opportunity presented to him, Root expects Broad to make the most of his chance, should he play at North Sound.
"No-one likes being left out. Especially someone as experienced as him,"Root said. "I expect him to come back and try and prove a point.
"He's someone who can be quite stubborn sometimes and likes to prove people wrong. That's a very good strength of his and something that can work in our favour."