In doing some research there's a chance this individual is in fact an Alston's mouse opossum (Marmosa alstoni).
When comparing to (Marmosa mexicana) Mexican mouse opossum 2014 - ZooChat the coloration of the fur on the top of the head and limbs seems darker and more brown. There's darker eye markings and the overall fur appears more woolly as well. There may be a little bit of a white tail tip here (not sure it's enough) but the banding on the tail here isn't found on the other animal. The two animals in question were spotted 30 minutes apart at 2 totally different locations around Manuel Antonio NP. I've always felt like both animals looked very different to begin with but wasn't sure if maybe this animal was a juvenile.
Fiona Reid's field guide to Mammals of Central America & SE Mexico states as the description:
"Largest mouse opossum. Upperparts gray-brown; underparts white or pale orange. Fur long and woolly. Prominent dark eye rings. Base of tail furred like the body for 25-50 mm; naked portion dark brown, with a long white tip (usually half brown, half white, sometimes all-dark or with short white tip or blotched toward tip)."
It could be but I wouldn't be placing any bets on that photo though, and I do think you're trying to force impressions onto the photo from a written description.
The tail appears to be completely washed out by the flash (compare to the branch it is sitting on), so I certainly wouldn't be using that as a basis, and I wouldn't even like to say whether the banding is real or an artifact.
The animal also does look like a juvenile rather than an adult.
You could email Fiona Reid and send her the photo to see what she thinks, but for myself (not having seen any of the species in real life) I'd want to be seeing the base of the tail to say which species it is.
