The Argentine international, who left Juventus as a free agent this summer, is yet to find another club and has been heavily tipped to move to Old Trafford.
And Ambrose believes that Dybala, who was Ronaldo’s teammate at Juventus, could partner the Portuguese in United’s attack but not replace him.
Ronaldo has told Manchester United he wants to leave Old Trafford following their inability to qualify for the Champions League but the Red Devils are reluctant to let the 37-year-old go.
“I think they should [sign Dybala] if Ronaldo stays. I don’t feel he [Dybala] is a suitable replacement for Cristiano Ronaldo,” Ambrose said on talkSPORT.