In reference to the patent: US8739921
is this patent only protected in the US?
Here is a generic process for checking. Note that there is some residual risk that data has not been entered correctly or that they filed a parallel application on the same date rather than a related application. Before taking big risks you should have a patent attorney augment your initial search.
Step 1 -- go to patents.google.com
Step 2 -- enter the patent number of interest.
Step 3 -- hit the Espacenet link in upper left corner.
Step 4 -- on the Espacenet page for this patent, hit the INPADOC patent family link in the left column.
Look at the results. The patent family will include related US applications, PCT applications, and foreign patents or and publications of foreign applications.
Keep in mind that for a PCT application that has not expired, the patent owner has an option to start seeking patents in 148 patent systems. While most systems place a deadline for entry at 30 or 31 months after the priority date, one can enter some systems later. The PCT deadline for entry into Canada is 42 months after the priority date for the first application in the chain. Another complication is that just because a patent application was published and shows up in ESPACENET does not mean that it went all the way to becoming a patent. Yet another complication is that the actual patent claims awarded may differ from country to country. Some types of claims are not allowable in some countries (methods of doing surgery for example).
In the particular case of US8739921 I do not see any connected cases showing up in ESPACENET, including a PCT case. Parallel filing US and non-US can be done but is unusual as you need a foreign filing license. Thus, the fact that there does not seem to be a related case to your patent of interest makes it unlikely that this inventor has rights in other countries.
You may find that others have obtained similar rights in other countries. So you should have a patent attorney in your market of interest verify that you have freedom to operate as planned without bumping into someone else's patent or to double-check that a data entry problem did not cause a failure in the patent family listing.
Please excuse the long answer but I hope that these posts can be useful to other users of the site.