Jesse Lingard accused Manchester United of making "false promises" to him during his final months at the club ahead of his first return to Old Trafford with Nottingham Forest on Tuesday.
Lingard came through United's academy, spending 22 years at the club before his departure on a free transfer at the end of last season.
The England international had been largely a rotation option during his time in United's senior squad, but he fell well down the pecking order in the 2020-21 campaign as he failed to make a single Premier League appearance in the first half of the season.
That led to him joining West Ham on loan and reinvigorating his career in London, scoring nine Premier League goals – a personal best for a single campaign – in 16 appearances.
Such form suggested Lingard still had a future at United and the club reportedly refused several transfer offers, but he remained a bit-part player in 2021-22; his 22 appearances across all competitions included just four starts.
His lack of minutes at the club during his final couple of years at United remains a source of bewilderment for Lingard, who will hope to prove a point in Forest's first league game since the World Cup.
"I don't know why I wasn't playing," Lingard told the Telegraph.
"I don't know what the problem was, whether it was politics or whatever. I still haven't got an answer to this day. I didn't even ask.
"I'd rather that someone, out of respect for me being there that long, told me 'this is why you're not playing', but I never got that.
"It was false promises. I was training hard and I was sharp, I was ready to play those games. When you're working hard in training and don't play at the end of it, it's very frustrating.
"I had to grind out that last year because I knew I'd be [leaving] on a free [transfer].
"I'm not really the type of guy to sulk. Around the dressing room, I'm always that bubbly character with good vibes.
"I was getting on with my training and doing my work, then here and there I'd come on and try to give my best. The last year was difficult."
Despite the frustration Lingard holds on to regarding his final years at United, resentment does not appear to have taken root.
Generally regarded a popular figure in the dressing room before he left the club, Lingard is not going to Old Trafford on Tuesday with revenge on the mind.
Instead, he is looking forward to finally being able to bid farewell.
"It will be an enjoyable one, but we want to win the game," he added.
"You can't get caught up too much emotionally, [but] it's a chance to say my goodbyes because I never really got that."
Forest go to Old Trafford second from bottom in the Premier League with only 13 points, while United will be hoping to reduce the gap to the top four back down to one point.