The 32-year-old midfielder, who was signed from Thai side Insee Police United, has yet to complete the necessary paperwork for his transfer to be made official.
And Home coach Lee Lim-saeng confirmed in Friday's pre-match press conference at the Jalan Besar Stadium that Wilkinson will be ineligible to play in the Charity Shield, while Singapore international Qiu Li is also a major doubt.
"We have a big problem [for Sunday's match] - John Wilkinson cannot play," Lee said.
"I [also] want to protect Qiu Li [as he is not fully fit]."
Defending S.League champions Tampines could also be without a key player as coach Steven Tan is faced with the dilemma of fielding star striker Aleksandar Duric, who is currently nursing a toe injury.
"We have a couple of injury doubts, including Aleksandar," Tan revealed.
"We may not want to risk him, especially because we have a tough game in Brunei [against DPMM FC] next weekend, so we'll see how it goes."
Nonetheless, the Tampines tactician is confident Sunday's game will prove to be an intriguing one, and believes his players are ready after a hectic pre-season programme.
"Basically we just came back from a training tour in Bangkok, where we had four tough games," Tan added. "We tried our various formations and tactics.
"Hopefully the players can gel as a team and I think we'll be ready for Sunday. Playing against Home United is always an interesting game."
Duric echoed his coach's sentiments, suggesting the game could be important for Rovers to assess where they stand ahead of the season, against a side many believe will challenge them for the title.
"The game may still be a friendly, but we want to win because we want to see where we stand before the season starts," Duric said. "It's important to see our current standing for the rest of the season."
Duric's international colleague Shi Jiayi, who captains the Protectors, is aware Sunday's game will be a tough test despite its non-competitive status.
"This is a good game - to play against Tampines - they always provide a tough game," Shi said.
"Hopefully we can perform well."
There will be plenty of pressure for the S.League to deliver this season, especially with plenty of attention being paid to Singapore outfit LIONSXII's participation in the Malaysia Super League, but Tan believes the onus is on the clubs to bring in the crowds.
"The game is very important because I think a collective effort is needed to make the S.League succeed," Tan added.
"We have to work hard and play good, attractive football to get the fans back. I think if we can get the fans back, we [the S.League] will do well."
In order to bring in the crowds for Sunday's match, the S.League have decided to waive all admission fees, while there will be pre-match entertainment, including a performance by 2009 Singapore Idol runner-up Sylvia Ratonel. A lucky draw will also be held where fans stand to win many lucrative prices.