Coutinho has again been linked with a move to England after struggling to live up to his club-record price tag at Barcelona and now unlikely to stick at Bayern Munich beyond the end of his loan spell.
Ferdinand says Coutinho, who turns 28 years old in June, still has quality to give, but United should forget about signing him as they have a better player in Bruno Fernandes.
"At Liverpool he was an amazing player, but since he left Liverpool he hasn't really done it yet," Ferdinand said during a Q&A on Instagram. "He struggled at Barcelona, struggled to fit in there and then he hasn't done amazingly well at Bayern.
"I think he is a great player, a huge talent, I think he needs saving. Coming into the Premier League to the right team would be a saviour for him - he needs that. I would have said Man United before Bruno Fernandes turned up, but not now.
"Would he improve Spurs? Yes he would, 100%. People like [Harry] Kane, Dele [Alli], [Heung-min] Son etc. all feed off someone like him. I think he would be a great addition to Spurs' squad.
"Arsenal he would definitely improve them, but where does he play in their system? They've got [Mesut] Ozil. I like Ozil if he is played well.
"Barely any team can afford him where he would fit in. He wouldn't fit into City, Liverpool, not any teams like that. Would he be a good addition, yes? Not Man United though, and he played for Liverpool so it is a difficult one. The rivalry is mad."