But Gloucester will field Vainikolo for the Ulster game, which comes just 24 hours before he meets up with England's RBS 6 Nations squad at Twickenham.
Vainikolo said: "I have been fortunate to have received a great deal of support, but I am really happy to be back, be around the boys again and thinking about rugby.
"It is great I have an opportunity this weekend, and I want to go out and repay the faith people have in me.
"There is a job to do on Sunday and we must make sure we achieve it."
With the juggernaut wing absent, Gloucester lost their last two games - against Guinness Premiership opponents Bath and Heineken Cup rivals the Ospreys.
Victory on Sunday, though, would see the Premiership leaders secure a home quarter-final.
Gloucester head coach Dean Ryan said: "Lesley has obviously been through a very tough time.
"But he is a very positive guy and I am sure he will channel it into a performance.
"He handles himself very well and is really keen to get back on to the pitch with us."
Vainikolo is among six changes from the side thumped 32-15 by the Ospreys in Swansea last Saturday, as Ryan also hands starts to fly-half Chris Paterson, scrum-half Rory Lawson, prop Nick Wood, hooker Olivier Azam and flanker Alasdair Strokosch.
Peter Buxton, meanwhile, moves from back-row to second-row duties as replacement for broken arm victim Alex Brown.