The 33-year-old was injured at Colchester police station in Essex during a struggle when he grabbed hold of a toilet bowl in his cell.
The Independent Police Complaints Commission (IPCC) has started an investigation into the incident.
It is believed seven Essex Police officers will be questioned.  The force has not commented.
It is thought the IPCC will investigate claims the man was handcuffed after his fingers were severed, and allegations of a significant time delay in taking him to hospital.
The prisoner, who lost part of three fingers on his left hand, was taken to hospital following the incident on 1 May.
He had been taken into custody in connection with a public order incident.
A spokesman for the IPCC said: "The Independent Police Complaints Commission is to independently investigate an incident at Colchester Police Station on May 1, 2015 where a man suffered serious hand injuries while in custody.
"Further information is likely to be published in due course."

In July, a major bug was discovered in the software that could let hijackers access data on up to a billion phones.
Manufacturers have been slow to roll out a fix because many variations of Android are widely used.
One Android expert said it was "about time" phone makers issued security fixes more quickly.
Android has been working to patch a vulnerability, known as Stagefright, which could let hackers access a phone's data simply by sending somebody a video message.
"My guess is that this is the single largest software update the world has ever seen," said Adrian Ludwig, Android's lead engineer for security, at hacking conference Black Hat.
LG, Samsung and Google have all said a number of their handsets will get the fix, with further updates every month.
Android is an open source operating system, with the software freely available for phone manufacturers to modify and use on their handsets.
The Google-led project does provide security fixes for the software, but phone manufacturers are responsible for sending the updates to their devices.
Some phones running old versions of Android are no longer updated by the manufacturer. Many companies also deploy customised versions of Android which take time to rebuild with the security changes.
Apple and BlackBerry can patch security problems more quickly because they develop both the software and the hardware for their devices.
BlackBerry's software is reviewed by mobile networks before being sent to handsets, while Apple can push updates to its phones whenever it wants.
"The very nature of Android is that manufacturers add their own software on top, so there have been delays in software roll-outs," said Jack Parsons, editor of Android Magazine.
"In the US it's even worse because mobile carriers often add their own software too, adding another layer of bureaucracy holding up security fixes.
"There's no real villain here, that's just how the system works. But there will always be security concerns with software, so it's right that some of the manufacturers are stepping up to deal with this now."

The Shoreham footbridge, which will link the two sections of the town over the River Adur estuary, will cost £9.8m instead of the estimated £5.8m.
Construction was supposed to have started in the spring, but will now get under way at the start of August.
The bridge is expected to take 45 weeks to complete and open to the public in the summer of 2013.
Funding has come from West Sussex County Council and sustainable transport charity Sustrans, which received a £50m grant from the Big Lottery Fund to extend the National Cycle Network.
The work was delayed to allow a review of the project by the council and the main contractor, Osborne.
Peter Wanless, chief executive of Big Lottery Fund, said the bridge would encourage people to make journeys on foot or by bike instead of by car.

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Blair Alston opened the scoring for the Bairns on the brink of half-time, heading an Aaron Muirhead cross into the bottom corner.
John Baird netted his 18th of the season shortly after the break.
But Derek Lyle pulled a goal back with a neat turn and finish and Iain Russell's strike levelled the scores in stoppage time.
A police force is being investigated after a prisoner had three fingers severed while in custody.
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Samsung, LG and Google have pledged to provide monthly security updates for smartphones running the Android operating system.
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Work to build a West Sussex footbridge which was delayed after the cost nearly doubled is to begin later this summer.
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Queen of the South fought back from two goals down to draw with promotion-chasing Falkirk in the Championship.