The Indian duo of Sriram and Shrinivas, currently serving as the Bangladesh T20 coach and performance analyst respectively, worked with the pacer to identify his mistakes.
Bangladesh's fast-bowling coach Allan Donald has credited Sridharan Sriram and Shrinivas Chandrasekaran for helping their struggling pacer Mustafizur Rahman get back his lost sting and play a key role in their ODI series win over India. (More Cricket News)
The Indian duo of Sriram and Shrinivas, currently serving as the Bangladesh T20 coach and performance analyst respectively, worked with the pacer to identify his mistakes.
"Sriram and Shrinivas spotted that in the release point of his slower ball, his palm was facing the batter," Donald told ESPNCricinfo on the sidelines of the opening Test that India won by 188 runs.
"It needed to be more side-on so that the palm was facing the off side on his point of release. He bowled a lot of overs to get that ball to come out softer and with a bit more bite. He worked really hard on nailing yorkers.
The 27-year-old, who is the most experienced among the Bangladesh pace battery, has been under pressure to deliver.
"He is a class act. When we needed a guy to step up against a world-class player in Rohit Sharma, Fizz came to the party and produced a great yorker to win the series for us. I love working with him. Mustafiz has been around the world. You write him off at your own peril. I know how hard he is working right now."
Having joined the side in March, Donald has overseen a rise in Bangladesh's pace bowling, after they relied on spinners for years. They started to turn it around with Courtney Walsh at helm, followed by Ottis Gibson.
The South African great took it from there as the Bangladesh bowlers reached the mark of 167 wickets for the year, their best across formats in a calendar year in over two decades of playing international cricket.
Ebadot Hossain's six-wicket haul paved the way for their historic Test win in Mount Maunganui, while they won an ODI series in South Africa where Taskin Ahmed was adjudged the Player of the Series.
"I coach mindset. I can be technical if I want to, but at this level, you can have the best plans in the world, but you must have the desire and the mongrel in your belly, you won't get past anyone.
"I think slowly but surely we are starting to see this growing in this group. This excites me," the 56-year-old said.
Terming the growth as significant, Donald said: "I have taken out the fear factor of making mistakes... I say it almost every day: don't worry about what I think sitting in the stands.
"I have been there. I have experienced your heartaches and mistakes. It is absolutely okay to make mistakes, as long as we give it a 100% crack out there. It doesn't matter if you go for six."
Changing the "conservative approach" has been the key, he said.
The second and final Test against India begins at Mirpur from Thursday.