The 25-year-old centre helped the Blues to a sixth place finish in the RaboDirect PRO12 last season but has been absent of late after limping out of the 28-9 defeat of the Dragons on December 23.
Roberts is certain to be in Warren Gatland's squad for the Six Nations, with Wales beginning their campaign against Ireland in Dublin on February 5.
The Blues only game before then is the must-win Heineken Cup clash at home to Racing Metro 92 on Sunday but Baber accepts Roberts knee is unlikely to have recovered in time.
"The knee injury has just plagued him a little bit longer than we thought," he said.
"Unfortunately I don't think he's going to be able to win the race to get himself into a position to train this week and partake in the game on the weekend.
"There's a little break between our last European game and the Six Nations..
"But that will be up to the Union obviously when he joins up with them to see how he progresses."
One man who will pull on the Blues shirt next weekend is full-back Leigh Halfpenny who drew praise from Baber for filling Dan Parks' kicking boots in the 15-22 win over London Irish.
"We've got two world-class kickers on the field," added Baber.
"When you're making decisions on who's going to kick and you speak to the players, Dan is the first to put his hand up to say Leigh has been doing a good job.
"That's the sort of pressure you want. Like any position, you want these boys to pressurise each other and get the best out of each other."
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