O'Boyle had an operation on Thursday to repair the damaged hamstring and will be out of action until January.
The 24-year-old Riordan struggled for game-time with Connacht last season, but did feature for Ireland at the Rugby World Cup Sevens tournament in Dubai in March and also lined out for Buccaneers in the AIB League.
Munster team manager Shaun Payne said: "Danny has fitted in really well. He'll be with us until the New Year at least.
"We'll see what happens, but he will get an opportunity with us."
The Munster squad enjoyed a change of scenery with a training run-out in Clonakilty last Friday, and some fun-filled surf lessons on the Inchydoney beach.
They are jetting away to Vilamoura in Portugal on Wednesday for a seven-day training camp.