Masi suffered damage to both his MCL and PCL in the 26-22 Aviva Premiership defeat to Sale Sharks at the AJ Bell Stadium on September 20 and hadn't featured until the weekend. He had surgery to repair the damage and wasn't expected to be fit again until mid-late February.
The 32-year-old's availability is a major plus for Pool 4 winners and tournament second seeds Wasps, particularly as they have been hit hard when it comes to backline injuries, with the likes of Ben Jacobs, Tom Varndell, Christian Wade, Hugo Southwell and Jack Moates all on the treatment table.
"He's come back weeks earlier than what we expected and that's testament to his character," defence coach Brad Davis told the Bucks Free Press.
"It's nice to see him out on the field. It gives us a few options on what we can do with the back line. (Backs coach) Stephen Jones' hands have been tied for much of the season with the injury setbacks we've had with our wings and our centres, so it's good we're getting a number back.
"Hopefully we can give him a little bit more game time next week and then he can put his hand up for selection for that block of four Premiership games."
Masi, who has won 80 caps for his country and was the RBS 6 Nations Player of the Championship in 2011, played 20 minutes at the weekend and is expected to add to that tally at Gloucester on Saturday.
Wasps then have six league fixtures before returning to Amlin Challenge Cup action against the Cherry and Whites at Adams Park on the first weekend of April, by which time Masi should be back to his best.
Sheehan, meanwhile, has signed on as injury cover until the end of the season, bringing a vast amount of southern hemisphere experience to a side chasing a second Amlin Challenge Cup crown and a top-six place in the Premiership.
The 34-year-old moves to High Wycombe from Western Force, having also played for the New South Wales Waratahs and Queensland Reds and won seven caps for the Wallabies, the last of which came on the 2012 end-of-year tour to Europe.
"We have been operating with only two first-team scrum halves in recent weeks, which clearly isn't enough. To be able to bring in someone with the experience that Brett has is brilliant," said Wasps boss Dai Young.
"We have two talented young scrum halves here in Joe Simpson and Charlie Davies so Brett brings a great balance to that group and I have no doubt that his experience will be a benefit for those two, as well as Jack Moates as he continues his recovery from injury.
"Having played all of my club rugby in Australia, it is great for me to be able to go overseas and experience a different league and a different brand of rugby," added Sheehan.
"Since the move was suggested to me, I have been impressed with what Dai Young and his team are doing at Wasps. The young players they have at the Club are very exciting and I hope that I can impart a bit of my experience during my time there; I can't wait to get going."