You've forgotten some very important details. Whose home turf are they on? Who has longer supply lines to worry about? What is the terrain like? Is it just the Mongols vs the Romans, or is one side fighting another war on a different front, as this could be common at the time?

Are we to assume this is one massive pitched battle on neutral ground?

Sorry, but your submission has been removed because we don't allow hypothetical questions. This sort of thing is better suited for /r/historicalwhatif.