Newcastle legend Shearer became the Premier League's all-time record goalscorer during his spell on Tyneside and also made the history books at club level.
Having netted 206 times in 405 games for the Magpies over the course of a decade, Shearer's heroics will not have gone down well at United and on Tyneside.
Sunderland, of course, are Newcastle's fiercest rivals while the Red Devils tried - and failed - to sign Shearer twice. Indeed, they swooped in 1992 but lost out to Blackburn and were snubbed again in 1996, the frontman instead moving to St James' Park.
And while speaking on The Rest is Football podcast, Shearer again proved his undying loyalty to Newcastle with a brutal swipe at two of their biggest foes.
Newcastle legend Alan Shearer aimed a brutal swipe at Manchester United and Sunderland
Guesting alongside co-hosts Lineker and Richards, Shearer was asked which team from any era of English football he would have liked to play for.
The supporter who sent in the question then cheekily added: 'PS, Alan, please say Manchester United, even if it is through gritted teeth.'
But Lineker would not let the matter rest and asked: 'Off the bat of that, if you had to sign for one, Manchester United or Sunderland, which would it be?'
Shearer holds no regrets over his choice of clubs, however.
Speaking before his retirement in 2006, he told The Sun: 'Of course, I was tempted to join Manchester United, but I do not regret either decision.
'I had a magical time at Blackburn, winning the league. And I completed my own dream of playing for my home town club of Newcastle.
'I have memories I will hold forever and a goalscoring record that makes me extremely proud.'