Don't want to put two and two together, but "long term niggle" would fit with with a stress fracture of a middle metatarsal, which are usually the result of ongoing wear and tear, and not one particular impact, like the outer ones.
If that were the case, it's very hard to estimate recovery time, as each case can vastly differ, but standard is at least 6-8 weeks complete rest.
There have been cases of players being out for 4, 5, even 6 months with metatarsal injuries, but those are rare cases, and most metatarsal injuries suffered in football seem to be related to the little toe or second toe.
Here's hoping it's none of the above, but he certainly hasn't been at his devastating best over recent weeks, I had thought that might be down to fatigue, or just a general quieter patch of form that all players go through, but some sort of ankle or foot injury would make sense.
