It's only a red flag if he doesn't look at the context though, and really even if we do scrape in he must know that this team isn't going to suddenly be a CL contender next season, so any player looking at joining United has to surely be looking long term.
My take is that Spurs and Liverpool might well beat us to top 4 this year, but if you are a player looking to win a PL title and be a serious contender in the CL, we are the better bet IMO over the long haul. I can only see CL as a deciding factor in not joining United if a player is going to Barca, Bayern, Real, juve or indeed staying at Atleti.
There is a difference between caring about playing in the CL and caring about actually being a contender for domestic and European titles, and you have to look beyond a singular season to gauge that.