I wonder if that would have much impact.
First, Islam was centered mainly around the West African cities. It was more of an urban phenomenon and the countryside was always much more pagan. And it was the countryside that had most of the population and was certainly the area where slavers got their slaves. And of course, Muslims aren't supposed to be selling other Muslims as slaves. Muslim slaves were not even exported to North Africa. So I think the pool of potential Muslims slaves were always going to be small.
Second, even if anyone was Muslim, they would likely not be literate. Someone who was literate in West Africa in that era would have to be a member of the elite, and therefore not really eligible for slavery. That means even though they are Muslim, they are probably very dependant on others for their religion even though they might know a lot orally. There are going to be lots of things that they don't know, and that is going to hurt being able to keep a pure Islam or pass it down through many generations.
Third, simply from a practical aspect, religion gives people a bond that allows them to act collectively. If your intention is to dominate and control people, you are going to do everything you can to prevent that. If Islam provides slaves a way to organize, the slaveowners will try to split up Muslims so they are less dangerous and keep them isolated among other slaves of different beliefs.
Fourth, for a long time slaves had a huge turnover due to dying. They died in less numbers than Indians or indentured Europeans, but there was a huge turnover. That makes a surviving Muslim community very difficult to establish.
Lastly, because of the above, I don't see Islam being sustained for a long period of time. Likely, any such belief will become corrupted and fall into a syncretic religion. We might see some voodoo deities that are based on Allah, Mohammed, or some angels. Or certain things corrupted into a Christian context once slaves were converted. There are going to be about 100-300 years of slavery, and that is a long time for illiterate people, with lots of turnover, and no form of organized community to establish/keep their own separate religion from the people who control them.
Also, 'Roots' is very entertaining, but it is not a true story. It is based on Alex Haley's researches, but it is fiction. Even some of the things Haley claimed was true, probably were not. Do not use it as a historical resource.
So as a thought experiment, you can imagine lots of things if the African slaves were dominantly Islam and kept their faith. But it's not really within the realm of plausibility in my opinion.