Parker joined Tottenham last season in a £6 million deal after four seasons with the east London club.
During that period he was named 'Hammer of the year' three times, and was also named footballer of the year by the Football Writers' Association in his final season even though the Hammers were relegated from the top flight.
The 32-year-old put in some lionhearted performances for the Hammers during his time at the club, but he was booed throughout his first appearance at Upton Park since departing for West Ham's local rivals Spurs.
The England midfielder, who gave away a penalty in Tottenham's 3-2 win, admits he was taken aback by the jeers from the home crowd.
"I didn't expect the West Ham fans to boo me and obviously it was a little bit upsetting," said Parker, who played 113 times for West Ham.
"I don't know why (they did it). I don't know if it's because I went to their local rivals... I'm not sure, but I just would have liked to think that maybe I would have got a better reception after what I did in my time here.
"I loved (Upton Park), the fans, and I loved the club.
"I had four fantastic years here and it will always be a club with a massive place in my heart. I was looking forward to coming back and I couldn't wait to go out there and play so I was disappointed (to be booed)."