Hmm - assuming obj, mtl and image (if textured) in the same directory and linking of the three files is as usual pointing to the current directory - I know about two issues where MM can't find the either the mtl or the image. In both cases MM loads the geometry but you get no or a red texture....
Would be interesting to see the content of the mtl. You can open it in a text editor.
Anyhow: You can remove the link from obj to mtl opening the obj in a text editor.
At top there should be a line like
mtllib ./someName.mtl
Remove this line to make the obj a standalone one. Note: MM now doesn't load UVs and if you save a result it will not have UVs anymore (which is ok if you do not need them further on). If you do one needs to repair the mtl.
