The warning comes with the Blues set to complete a permanent deal for the 25-year-old – with an announcement expected soon.
After ticking a new centre-back and striker off his January wish list, Danny Cowley’s attention has quickly turned to a new wing-back.
Stadium of Light academy graduate Hume has been the man identified to fulfil that role, after Lee Brown and Reeco Hackett’s struggles to fill the brief.
But former Sunderland favourite Phillips, who last week took charge of non-league South Shields, isn’t convinced it’s the right move for his one-time employers.
He believes the Black Cats’ promotion bid may suffer as a consequence of the transfer.
Lee Johnson’s side currently sit second in the League One table during their fourth season in the third tier.
And former striker Phillips thinks Sunderland should retain his services for their end-of-season run in.
Speaking to Football Insider, he said: ‘It is a very tough one. It’s about numbers.
‘There are still a lot of games to go and a lot of big games for Sunderland. Anything can happen.
‘Do you let a player go and run the risk of leaving yourself a little bit short?
‘They’re in such a good position right now. It is a difficult one.’
Despite signing a new contract on Wearside in the summer, Hume has struggled for game time this term.
The left wing-back has played a total of eight games in all competitions, with only half of those appearances coming in League One.
Meanwhile, he had to watch on from the sidelines as his current side beat the Blues 1-0 on Saturday to strengthen their promotion credentials.
Phillips continued: ‘Perhaps the player wants to leave, though, and if that’s the case it’s difficult.
‘If they can somehow keep him at the club and somehow keep him happy I’d keep him. I’d be very reluctant to let him go.
‘I would be cautious of numbers going forward. It will be interesting to see what happens this week.’
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