Umaga (37) moved to Counties-Manukau from the Toulon club in France and is the assistant coach for the Steelers.
His decision to come back to Counties-Manukau was seen as a coup for the province but he may not play in his new province's first game in the ITM Cup.
Steelers head coach Milton Haig said Umaga did not train yesterday and was looking doubtful for Saturday.
He was feeling the effects of two hard pre-season games.
"We're not really sure whether he will be right. He'll be the one who will be making the call on that. After all these years he knows his body better than anyone," Haig said.
"But we have to be careful with him and not rush him into things or we'll burn him out early and he'll be out for the rest of the season."
Counties-Manukau finished bottom in the competition last season but had some narrow losses, which Haig said came down to inexperience and giving away penalties at vital times.
"Certainly. we have to scratch up on our discipline around the ruck and keep hold of the ball."
Haig has a couple of potential match-winners in the likes of outside backs Tim Nanai-Williams, who is coming back from a leg injury, and Sherwin Stowers, who has been a revelation in the sevens game.
"Certainly, those guys out wide are the strength of our side but it always going to be difficult to get the ball to them if you don't win enough ball up front. We've just got to make sure we can capitalise on the opportunities."
Counties-Manukau drew 33-33 with Auckland in a pre-season match and then lost to North Harbour 22-21, though it scored three tries to one.
Injuries have hit the loose forward stocks hard, even before the season has begun.
Experienced No 8 Waka Setitaia has a knee injury, New Zealand sevens captain DJ Forbes is having a break after battering his body on the sevens world circuit, while openside flanker Mark Selwyn is struggling to overcome a leg injury.
The match on Saturday will be refereed by Vinny Munro.
