slumdog millionaire's dev patel nominated for sag award

Nominations for the 15th Annual Screen Actors Guild Awards were announced yesterday. Among the nominees, I noticed that Dev Patel was up for Best Supporting Actor for his performance in Slumdog Millionaire. Here's the full list of nominees: Nominees for annual Screen Actors Guild Awards.

That's great, and it's cool to see him acknowledged for his work in such a wonderful movie... but what's up with the supporting actor nod? Is he not the lead actor in that movie? I certainly thought he was. Does SAG have some kind of rule, maybe having to with screen time or something, that officially determines who can be up for lead or supporting? It just seems odd.