The 21-year-old was one of new Man United boss Erik ten Hag's top targets this summer, as he looked to rebuild the Red Devils squad.
However, he decided to stay with the Eredivisie champions and returned to training this week.
And Timber posted a picture of himself to Twitter with the Ajax badge in the background with the message: "Still here tho", followed by three red crosses.
Whether the post is a direct dig at United remains to be seen, but it certainly riled a few of the club's supporters.
talkSPORT's Man United correspondent Mark Goldbridge replied to the tweet, stating: "Should have backed yourself. You'd have been a starter at United,"
It is unclear as to whether his national team coach and ex-Red Devils boss Louis van Gaal had anything to do with his decision, after advising the defender against a move to his former club.
"I think a player with his [Timber's] qualities can play in the Premier League," Van Gaal said in a pre-match press conference earlier this summer.
"That's not a problem. But if he has to make this step now… that's another question. It is not so wise I think [to move to Man United]. He has to play," he added.
Ten Hag has now turned his attention to other targets, with Timber's Ajax teammate Lisandro Martinez on their summer shortlist.
And talkSPORT understands that, unlike Martinez, he has told the Dutch giants he wants to leave Amsterdam.
The Red Devils have already signed Feyenoord left-back Tyrell Malacia this transfer window, with more expected to come over the next few weeks.