Onions landed in South Africa on Wednesday and trained with the Dolphins on Thursday afternoon.
“We still have to see if he’s going to play tomorrow. He’s just got here, so he may need time to settle in,” Dolphins’ captain Morne van Wyk said in a statement on Thursday.
“He’s quite a new guy to me, but he comes highly-rated and highly-respected. If someone like Dale Benkenstein sings your praises then you’re obviously quite an accomplished cricketer.
“We just want to create an environment that he can enjoy playing cricket and enjoy his time in South Africa.”
The Durban franchise signed the fast bowler in September as one of their major recruits for the new cricket season.
Onions spent several seasons with Dolphins batting coach Benkenstein at Durham in England.