The former Liverpool and Scotland player is not convinced that the Red Devils are bidding for one of their top targets after reports of a £15million transfer from Juventus emerged.
Rabiot, 27, is available for a move from the Italians this summer and United are said to have lined up a deal as their hopes of signing Frenkie de Jong fade.
Souness told talkSPORT: “He was at PSG and ran down his contract and went to Juve. He’s not done it there.
“I’m not sure that’s much of an upgrade on what they’ve got there already.
“What it stinks of is if you are signing him at this time, he’s not your first choice. Maybe your second. It is 24 days to go until the window closes, one game gone, I think you are reacting to things now.
“That’s when you make mistakes.”