What are the needed physical characteristics of a bipedal dinosaur to be able to carry a human rider approximately as usefully as a horse?

Inspired by this drawing.. The pictured rider is technically supposed to be lighter than a human but carrying gear, armour, and weaponry so same difference.

I'm ok with both avian and non-avian dinosaur responses. Ostriches have been ridden throughout history but afaik only as a novelty. Best info I can find why not is this.

Assume: you have generational human time and can breed/select for any reasonable traits.