YES-it's a more than  7 day injury. Seven days off so reportable.

I had a similar one several years ago where a member of staff cut her hand while at work (work related so incident form submitted). At the time she didn't go home or attend hospital - simply shoved a large plaster from the first aid box on it and continued at work.

However, after a couple of weeks and still experiencing a lot of pain and tenderness in her hand she went to her GP who sent her up to the hospital for a closer look. They said she had managed to neatly cut the tendon and that it would require surgery. They admitted her the next day and she was signed off for 2 weeks - over a month since the inital incident. I submitted a RIDDOR report with full explanation of what had happened, when she had sought medical help etc when I was told by her line manager that she had been signed off. 

There was no follow up from the HSE so guess they were happy with the explanation. I'd report as absence attributed directly to the incident that has already been recorded as a workplace injury. They probably won't come back to you about it though but if the person later submits a compensation claim, you have the RIDDOR report to include with the paperwork to your company's solicitor at least.