With both players out of the tour, Ruddock will fly from Argentina immediately and will join the senior squad in Rotorua on Monday evening ahead of the game against New Zealand Maori next Friday (June 18).
The unfortunate Muldoon will undergo surgery on his broken arm in Auckland in the next 24 hours before returning home with Heaslip.
Second row Mick O'Driscoll suffered back spasms which forced him to leave the pitch during the first half of the All Blacks game. The injury has improved and he will continue to receive treatment over the next 48 hours.
Winger Shane Horgan has also recovered from the stomach complaint that prevented him leaving with the balance of the squad from Ireland and has arrived in Rotorua.
He joins hooker Jerry Flannery who flew out to meet up with the squad in New Plymouth following the calf strain he sustained in the Barbarians game.
Ireland head coach Declan Kidney said: "We are obviously disappointed to be losing John following his injury. I felt any replacement should be one that would benefit from the experience.
"There were other options in terms of back row replacements, but rather than bring in somebody who we were already aware of and what they could bring (to the squad), we felt Rhys would be a different option.
"He has performed very well for the Ireland Under-20s and with Leinster when he has had the chance to play this season, so this will be a good chance for him to gain experience and we have every confidence in him."