Muliaina, who will cover midfield, wing and fullback for the Chiefs, returns from Japanese club NTT Docomo Red Hurricanes, for whom he has played for two seasons following the All Blacks' World Cup victory. Depending on how long his club stays in the competition, he might not return to Hamilton until mid February. The Chiefs play the Crusaders on February 21.
Similarly, the jury is out on whether Fruean's heart issues will allow him to start that game against his former franchise.
He is running after his recent surgery on a faulty valve, but has yet to get into contact work.
Both players could be excellent signings for the defending champions, but there are risks attached. First among those is how Fruean copes in his comeback after his dramatic late-season withdrawal from the Crusaders and the second is how Muliaina copes with the physicality of the competition after two years in the less-taxing Japanese game.
Coach Dave Rennie said 33-year-old Muliaina, who played 100 tests, was in excellent physical shape after minor knee surgery.
"He reckons he's in the best condition of his life," Rennie said. "He's had a bit of a tidy up of his knee and hasn't played much this season but he's an absolute professional and we spoke to people in Japan who had massive wraps on him over there. He's played 100 tests and over 100 Super Rugby games and we think that experience will be invaluable for us."
Rennie said he had spoken often to Muliaina, who had made it clear that he was keen to return to his former franchise which has won two titles in his absence.
Sonny Bill Williams' unavailability had opened the door for both signings, Rennie said. Fruean, 25, who played alongside Williams at the Crusaders in 2011, has a similar physical presence.
"We were pretty keen to get a big midfielder ... we always knew if Sonny didn't come over we had an option to lure Robbie up and it certainly had a bearing on that."
Rennie said the Chiefs had investigated Fruean's heart problem. "We've got cardiologists reports and specialists reports. They've been really positive."
He added: "We're willing to be reasonably patient there. We've got a lot of cover in midfield."
Andrew Horrell, Charlie Ngatai, Tim Nanai-Williams and Bundee Aki are all midfield options, but none have the size or power of Fruean at his best.
Rennie said Tom Marshall, another who has moved from the Crusaders, was also capable of playing in the midfield. However, he is likely to prefer to play fullback which happens to be Muliaina's specialist position.
Fruean said: "I am thankful for the chance to return to rugby at this level and am really looking forward to the Chiefs environment and the beginning of a new and important chapter for me."
