Cornelio will be the Nationals' opener for Tuesday's game in Detroit, Mark Zuckerman of NatsJournal.com reports.
Cornelio was used as a starting pitcher some earlier this season in the minors and has recorded more than three outs in 13 of 16 appearances for the Nationals, so he'll be capable of covering multiple innings, if needed. Jackson Kent is poised to fill a bulk-relief role Tuesday.