Khan is not an elite 135 pounder yet IMO. Getting a questionable victory over a past prime Featherweight does not quality you. Diaz stops Khan early, just like all the other top 135 pounders.
Khan would win by late TKO.
Diaz would win the first few rounds, getting up close and roughing Khan up. After round 6, Diaz gets gassed and Khan stops him with a flurry of shots in round 9
Diaz would press him all night. Diaz doesn't have a lot of power but then again Khan can't take a punch for shit. Would be a good fight. Khan would probably catch Diaz a lot coming in, but Diaz would surely land a lot of punches with his flurries.