Rio Ferdinand has urged Manchester United to sign John Stones to bring much-needed defensive solidity to Old Trafford and believes the Everton centre-half would be foolish to join Chelsea.

José Mourinho’s club have attempted to prise Stones from Everton, who insist that the player is not for sale and have rejected two bids from Chelsea, but United are also interested and Ferdinand says his former club would be well-advised to weigh in with an offer.

The former England captain argues that any choice for the 21-year-old between joining United or Chelsea ought to be straightforward owing to the competition for places at Stamford Bridge. Ferdinand says the state of Louis van Gaal’s defence is such that Stones would immediately become its linchpin but that the former Barnsley player may not be ready to dislodge John Terry or Gary Cahill from Chelsea’s backline.

“If he goes to Man United, he plays every week so that’s the right club for him,” Ferdinand said. “If he goes to Chelsea, he’s not playing every week. He’s at that stage of his development where he wants to play every week. If you say to any young footballer to leave a club to sit on the bench, you’ve got to be a lunatic. You have to be.”

Stones has yet to play in the Champions League and has four England caps but Ferdinand sees qualities that persuade him the player is “the real deal”. Ferdinand was on the substitutes’ bench at Queens Park Rangers when Stones played there for Everton in March.

“He’s very good on the ball,” said Ferdinand, speaking at a BT Sport event to launch the broadcaster’s football coverage for the coming season. “I took a real interest in him and watched him at Loftus Road against Bobby Zamora, who gave most centre-halves problems last season physically, and he handled Bobby well, and that physical element. That was one of the areas that I hadn’t really seen him under pressure with and he looked up for that battle. With all the other stuff he’s got, he’s got a real good chance.”

Ferdinand is clear that United’s backline is in need of improvement. “I don’t think [Van Gaal] knows his best defence – that’s a problem in itself,” he said. “Part of the reason he doesn’t know it is because he hasn’t had a chance to watch them all play over a long time – consistently for 12 or 15 games. If you don’t get that familiarity between you as defenders, it’s difficult to build foundations.

“I played with [Nemanja] Vidic for six years with no fitness problems between us. I woke up on a Saturday knowing that Vida was going to be fit and he was the same with me. At the moment these guys aren’t doing that. They don’t know if Chris Smalling is going to be fit, if Phil Jones, Jonny Evans or Marcos Rojo are going to be fit. So how can you start to build relationships? That’s been part of the problem for Van Gaal.” United have spent much of this summer trying to reinforce their defence by tempting Sergio Ramos from Real Madrid but Ferdinand doubts whether that was ever likely to happen. “I always knew that was never a goer. I said at the beginning on Twitter that he’s obviously up for a new contract [at Real] and he may be using United in that respect. Whether that’s true or not I don’t know but he’s signing a new deal according to what we read.”

Given that Chelsea have had bids of up to £26m rejected for Stones, and Roberto Martínez reaffirmed on Sunday Everton’s determination to hold on to a player on whom they are counting heavily, it would probably take an exceptionally large fee to alter the status quo. Ferdinand, who moved to Manchester United from Leeds United in 2002 for a then world-record fee for a defender, says that would pose challenges for Stones in addition to the more intense focus that would come from moving to Old Trafford. “The difference is the mentality,” says Ferdinand. “Physically he knows he can do it but it’s mentally.

“Mine was different. I first went from West Ham to Leeds, not West Ham to Manchester United – to huge expectations of winning the league. Leeds were happy to develop and get in the Champions League and, wherever they got in that, it was a great bonus. So the pressure was different. He is not going to be afforded that. Huge price tag, he is not going to be able to go in under the radar and bed in.

“He’s got to ask himself: ‘Am I ready for that? Can I deal with that?’ Those are the questions he has got to ask. I don’t know him personally. Looking at him as a footballer, technically he plays with a certain amount of maturity; I think he would be ready to go and do that. But he is the better judge of that.”

Defence is not the only sector of the United team that Ferdinand has concerns about. He says the club looks set to be heavily reliant on Wayne Rooney for goals and is baffled by Van Gaal’s apparent reluctance to use Javier Hernández. “The pressure is huge for Rooney. He has got a big season ahead of him. He has to score 20-25 goals this season for United to win the league and be successful.

“In any capacity the No9 for United has to score 25 goals a season. He’s not on his own, he has [Memphis] Depay, [Juan] Mata and [Marouane] Fellaini chipping in but, if he gets injured, unless they go in the transfer market ... They have got Chicharito there and he is a fantastic player – how he is not a regular in that team baffles me. I think in terms of movement, when the ball goes wide, he has the best movement in the world. But he has not been afforded that confidence from the manager as yet to say: ‘You are my No9, you get me 25 goals this season, you are playing every game.’ And until he gets that, you will not know if he is capable of doing it.”

BT Sport will be showing 38 Premier League games exclusively live this season, starting with Manchester United v Tottenham Hotspur on Saturday