"It's difficult to know what's going on," former Arsenal defender Sylvinho told ITV1.
"Robinho is a good guy and it was a a good contribution from him.
"I believe he wants to stay because he loves Manchester City and he enjoys playing here. He's one of the best players in the world. If he stays here it will be good for us."
Martin Petrov opened the scoring for City before Paul Hayes levelled for the Iron.
Nedum Onuoha then put City back ahead before Sylvinho's screamer put the Premier League side 3-1 up.
A Dedryk Boyata own goal put Scunthorpe back in it before Robinho hit the winner.
"It was a great result for us. You have to beat these kind of teams to get to Wembley but I think the result is good for us," added Sylvinho.
City boss Roberto Mancini looked uncomfortable when answering questions on Robinho's future.
When asked if the striker would be staying at the club, the Italian replied: "I think so, because he's an important player. He scored a good goal today."
Scunthorpe boss Nigel Adkins admitted City were a class apart from his side.
"My team worked very hard for us today," he said.
"We tried to have a good go at them you can't intimidate them, their class was there for all to see."