A scan last summer revealed that he still has a piece of metal in his knee following surgery during his days at Inverness Caley Thistle, but it has not stopped Rankin putting in some match-winning performances this season.
The 30-year-old midfielder is now looking forward to today's league meeting with Celtic - he refuses to give up hope of his side overhauling Aberdeen and Motherwell to finish second in the league and qualify for next season's Europa League - and next weekend's Scottish Cup semi-final against the other half of the Old Firm, Rangers.
United are entering a crunch week which will define their season, but Rankin swears that fourth-top United are blanking out all talk of their cup semi-final showdown to concentrate on finishing as high up the table as possible.
He said: "A few people have asked, 'What about the semi-final?' But it hasn't even been discussed.
"There has always been something for us to play for and we now have a game against the champions, so the semi is not something we have had time to dwell on or think about.
"It has probably helped our form in the build-up, as sometimes you can get side-tracked by the prospect of a semi-final. When you get to this stage of the campaign boys begin to switch off, but we can still finish in second place and make a cup final."
Regarding his knee, Rankin said: "There is a bit of metal left in from the first time I had an op on it at Inverness. It has been there for years.
"I didn't know it was there until I went for my scan last year. I'm not sure how or why it is there, but when they operated last summer they just decided to leave it in as it is not causing me any problems.
"To be honest, when you have a wee niggly injury like I had, you are trying to play through it until you can have a tidy-up. Last season, I couldn't turn over my right shoulder. I would just feel it click, so I always moved to my left. I am moving a lot freer this season, so it was definitely the right decision to have the tidy-up.
"The metal will be left there as it is now part of me, but I might have to have a rethink after the summer if I am getting beeped going through airport security when leaving on holiday. Seriously, it is not something I am worried about."
Rankin's manager, Jackie McNamara, insists previous encounters with Celtic this season proves his players can live with the best. He said: "We were unlucky not to beat them at Parkhead before Christmas and earlier in the season we lost to a last-minute free-kick.
"This is another big game which we're looking forward to and there's still plenty to play for between now and the end of the season."