The UFC revealed the news on Wednesday evening and stated that Rocha had suffered an undisclosed injury during a last minute training session in Baltimore.
The last minute cancelation is a tough break for Ellenberger, who was set to make his long-awaited UFC debut. His original opponent was Yancy Medeiros, who pulled out of their fight in favour of a main card match-up with Jim Miller when Bobby Green withdrew from that match-up.
Ellenberger and Rocha will both be paid their show money and "Excalibur" will continue to cut weight to remain an alternate on the card in case another late replacement is required.
UFC 172 marks the first 10-fight PPV card since UFC 150. The event takes place Saturday night at the Baltimore Arena in Baltimore, Maryland.