The two famous rugby institutions clash for the first time in Hong Kong on June 1.
Rees was a Test starter for the Lions in 2009 but four years on he will turn out in the black and white hoops of the Baa-Baas and he admits there will be players determined to make a statement.
"There's a few Welsh players here... who have probably missed out on the Lions tour," said the 32-year-old.
"We've got to hold our hands up and be counted, and there's no better way than playing the Lions in Hong Kong."
The Barbarians face England at Twickenham on Sunday, with Rees in the starting line-up.
They then jet to Hong Kong to face Warren Gatland's Lions. And after missing out on selection for the 10-match tour Rees has set himself the goal of making the 2015 World Cup.
"The 2015 World Cup, it's not far away and that's probably my own personal challenge," said Rees.
"The game is going to be intense, there's going to be a few other Welsh boys who'll be putting their hand up as well, and it's great to have that opportunity to play against the Lions.
"It's a great honour to be asked to represent the Baa-Baas and we start off with a tough test against England at Twickenham."
The hooker will join Cardiff Blues in the summer and he his relishing his reunion with Gethin Jenkins, who returns to the capital city region.
"I've been with the Scarlets for nine years and I've had some great times there but I felt at this time in my career I needed a new challenge," added Rees.
"Obviously I've worked with [Blues director of rugby] Phil Davies in the past and he's been a great coach - when he was at the Scarlets he was probably the making of me in terms of giving me an opportunity.
"Obviously there's a few other signings he's made with Gethin [Jenkins] coming back from Toulon, and hopefully we can strengthen the squad, put in a good season and hopefully win some silverware."