But the veteran stopper insists he has been overwhelmed with messages of support after signing a short-term deal with the Black Cats.
Harper - who spent 20 years at Newcastle - was a free agent after leaving Hull last summer and reveals he turned down offers from clubs throughout the divisions in England and also abroad.
Sunderland boss Sam Allardyce - who worked with Harper at Newcastle a decade ago - is understood to have mooted the possibility of joining the Black Cats last month and then made fresh contact after the departure of Costel Pantilimon.
"It was out the blue," revealed Harper.
"I got a phone call on Thursday afternoon from the gaffer who said, 'how are you?' I said I am still in decent nick and he asked if I fancied coming in for a chat, which I did, and the rest is history.
"There was always going to be a reaction but I am not the first, I certainly won't be the last and bigger and better players than me have done it and played for both clubs with no issues.
"I think I am the first to do it in the new social media age so I read the script and deactivated my Twitter account before I signed so as not to get involved.
"But every message I have had from friends, ex-colleagues and even genuine, hard core Newcastle fans have said: 'great to see you back in the game and good luck'.
"I had a decent Christmas and New Year but at the start of January I have just battered the gym for the last three weeks, not knowing anything was going to happen.
"It turned out to be a good decision."
Ex-Newcastle keeper Harper has agreed a contract at Sunderland until the end of the season, to act as back-up behind Jordan Pickford and first-choice stopper Vito Mannone.
Sunderland face Manchester City, Liverpool and Manchester United in their next three Premier League games and Harper says Allardyce's squad will have to draw on all their recent experience of beating the odds.
And the 40-year-old believes the Black Cats are battling against relegation with five other sides, with Aston Villa, Newcastle United, Swansea City, Norwich City and AFC Bournemouth all in danger.
"The next three are pretty tough," said Harper.
"After that there is a decent run of about seven or eight games.
"The club had a history of doing well against the big boys and we need to do that with the next three games.
"Teams can easily get dragged into it.
"Sunderland have a recent track record and experience of pulling clear from that and it was tough for Norwich to score four and get beat [by Liverpool].
"Chelsea are out of it, Swansea got a good result.
"But there are still half a dozen teams still very much in it."