Chris Burke could be sidelined for a month after he was hurt in a challenge by former Rangers team-mate Fernando Ricksen, now of Zenit St Petersburg.
Fernando Ricksen (centre) helped Zenit defeat Rangers
Rangers lost 4-1 at Ibrox in a friendly arranged as part of Ricksen's season-long loan deal to the Russians.
Burke spent a month out with an ankle injury picked up in pre-season.
And manager Paul Le Guen said: "I am worried about Burke because it is the same ankle. He worked to get back, but I am worried."
The friendly was marred by Ricksen's wreckless challenge, which left Scotland winger Burke to crawl off the field in agony shortly after being introduced as a second-half substitute.
"He is in less pain than when he was in South Africa - but, as it is the same ankle, I am worried," said Le Guen.
Fernando would never do that on purpose to a Rangers player
Zenit St Petersburg coach Dick Advocaat
"These things can happen and I have no problem with Fernando.
"We lost 4-1, but I am very worried about Burke not about the result.
"We will find out on Thursday. I hope he is out for less than before, but it is too early to say."
Former Rangers manager Dick Advocaat, now in charge of Zenit St Petersburg, also insisted Ricksen was not to blame.
The former Netherlands boss took Ricksen to Zenit on loan following his breach of discipline on board a club flight to South Africa.
"Fernando did a good job at Rangers and I needed more physical players," he said. "He has a temperament I like.
"It was not Fernando's intention because the relationship with the players is still very good.
"These kind of things can happen in football and, unfortunately, it is just Fernando.
"He would never do that on purpose to a Rangers player because he is still owned by Rangers, so he would never do that."
Ricksen had earlier received a warm reception from the Ibrox fans but was substituted by Advocaat shortly after the tackle.
Rangers had fielded a side made up largely of fringe players and recent signing Filip Sebo missed two early opportunities before Zenit took the lead.
Lee Ho rose unchallenged inside the penalty area to direct a header from Radek Sirl's free-kick beyond Allan McGregor.
Zenit, currently ninth in Russia's top flight, doubled their advantage eight minutes after the interval.
Igor Denisov and Ricksen played a one-two and Denisov was allowed to advance unchallenged to the 18-yard box before rifling a shot beyond McGregor.
But Rangers had halved the deficit as Nacho Novo played in Gavin Rae to slot past Kamil Contoalsky.
Zenit again cut through the Rangers defence far too easily as Alexander Kerzhakov slotted home the third.
Oleksander Spivak smashed in an 18-yard angled drive with seconds remaining.