England know they can't afford to take Bangladesh lightly but also need to keep players fresh
After winning the first Test by just 22 runs, England know they can't afford to take Bangladesh lightly but also need to keep players such as Broad fresh for a five-Test tour of India immediately afterwards.
The 30-year-old was in line to have made his 100th Test appearance in Dhaka but will now have to wait until he gets to India to achieve the landmark, with Steven Finn called up in his place.
With Jimmy Anderson currently sidelined, Broad is England's senior bowler but he had little joy from what was a spin-friendly pitch in the first Test in Chittagong and took just two wickets.
"I'm sure he'd have liked to have got that milestone but this is a case of looking a bit further to the future and he understands the amount of cricket he could play over the next six weeks," Cook said of Broad.
"We want other people up and running. We don't want to go to India with too many people not playing any cricket."
The 24-year-old Ansari has been handed his chance to impress in place of Gareth Batty who has been left out after being recalled for the Chittagong Test for the first time in more than a decade.
Cook said it was a tough call to omit the 39-year-old in place of Ansari, a slow left armer who is also a decent batsman, but that it was too risky to go into the match with four spinners and only two seamers.
"I thought Gareth got better and better as he settled into it," said Cook.
"In an ideal world he'd probably like another game but we liked the three spinners-three seamers option and when the ball started to reverse we saw how dangerous our seamers were.
"So we want that option and did want to give Zafar a go."
Cook said the selectors had also considered replacing the allrounder Chris Woakes with Jake Ball, but in the end decided that Woakes needed more playing time in the build-up to the India series.
"In an ideal way we would have liked Jake to have a game as well," said Cook.
"But we feel as if Chris Woakes hasn't played a huge amount of cricket on the subcontinent, didn't have a huge workload in the last game and we feel as if it'll be beneficial for him to experience in these conditions."
Woakes bowled just 14 overs in Chittagong but made useful contributions with the bat, scoring 36 and an unbeaten 19 in his two innings.