The 37-year-old forward was absent from the Red Devils' pre-season tour. Sky Sports have reported that the Portuguese wants to leave the club in search of Champions League football.
However, Ferdinand has claimed that Ronaldo's departure would leave Manchester United incredibly light up front. He told Sport Bible:
"After this season, I think it'll be a good question to answer. I think he's still in the process, he's still at the club. Last season, top goalscorer and I think the way that this has played out in the pre-season, the club aren't letting him go nowhere. Says to me, the club are thinking, 'Well where are we going to find 20 goals? If we let him go, where are the 20 goals coming from?'"
"It's up to the manager to make a decision, 'Can I bank on Rashford, Martial, Sancho, Elanga to fill that void?' If he makes the decision that they can, then great, go for it."
The former England defender added:
"I wouldn't risk it. Martial hasn't proved it yet consistently enough, he was allowed to go on loan last season. So that tells you that he's somebody they're not 100% sure about. He hasn't done it consistently enough."
"Rashford last season wasn't consistent, Elanga is a young kid, Sancho's last season was his first season. They've all got something to prove this season which is a good thing, I think. But you've got one man in the building if you put him on the pitch, he scores goals."
Who can step up to replace Cristiano Ronaldo at Manchester United?
The five-time Ballon d'Or winner played his first match for the Red Devils in 12 weeks over the weekend. He featured in the first half of United's 1-1 friendly draw with Rayo Vallecano on July 31.
Ronaldo looked way off the pace, with Erik ten Hag's system demanding a high level of fitness as well as his forwards pressing from the front.
With that in mind, Manchester United's centre-forward hopefuls have a huge opportunity to stake a claim to become the club's first-choice striker. Edinson Cavani left at the end of June, which has led to Antony Martial leading the line during pre-season friendlies.
The 26-year-old was sent out on loan to Sevilla during the second half of last season, where he failed to score a single goal in La Liga. This latest opportunity is surely the Frenchman's final chance to make an impression at Old Trafford, having been at the club since 2015.