United executive vice-chairman Ed Woodward is set to appoint Solskjaer on a permanent basis this summer.
The Red Devils were expected to bring Tottenham’s Mauricio Pochettino to Old Trafford.
However, Solskjaer’s remarkable turnaround at United has seen him leapfrog Pochettino as the favourite to land the job.
United are expected to splash the cash this summer as Woodward eyes a Premier League title charge next season.
However, Castles says United’s approach shouldn’t be Solskjaer telling Woodward what players he wants as the Norwegian has a poor record in the market.
“[Solskjaer] desperately wants the job, he was the outsider when he was appointed, he now becomes the fans’ favourite,” Castles said on The Transfer Window podcast.
“But he will clearly not produce the kind of demands on the club that [Jose] Mourinho did, or as you would expect a coach of Mourinho’s standard or status if he was to be hired in Solskjaer’s place to make upon the club.
“So I don’t think the solution is gonna be Solskjaer saying: ‘Ed, I’m not happy with what the scouting team’s proposing. I want to go for this player instead.’ And Solskjaer pushing and pushing and pushing until he gets the right player in.
“And one other further point in this is, his own record in the transfer market as a manager is really poor.
“If you look at what he did when he came in at Cardiff, the money that was spent, significant sums for Cardiff City to try and keep them in the Premier League on Norwegian players with direct links to his own agent who played very few games for the club. It’s not impressive.
“So in summary, I would say I don’t see this problem going away. I think it’s structural, it’s fundamental at Manchester United.
“If they get the right director of football, they can’t start working on it. But it’s mid-March already.
“We’re already two months beyond when well-organised clubs have got their recruitment team and their recruitment strategy in place for the next season.
“And United are talking about, albeit they’re looking at players, albeit they’re having discussions with Solskjaer about who he thinks should come in, they’re talking about bringing in a director of football during that period, which is, for well-organised clubs, the most important part of their recruitment window.”