Thomas Tuchel vowed to never give up on Saul Niguez after Chelsea's midfield selection issues deepened with the loss of Mateo Kovacic to coronavirus.

Kovacic returned to training on Monday following a six-week injury lay-off and was expected to be part of Chelsea's squad to face Zenit on Wednesday.

However, Kovacic returned a positive COVID-19 on Tuesday and will miss the Champions League trip to Russia, while he is also set to sit out the league games with Leeds United and Everton.

Chelsea are without fellow midfielders N'Golo Kante and Jorginho, plus defenders Ben Chilwell and Trevoh Chalobah, and Tuchel is cautious about overworking his players.

"We had a bit of an acute overload due to injuries and the schedule," he said at Tuesday's pre-match news conference.

"Due to injuries in some positions, like central midfield, we have struggled. The game tomorrow is not the problem, it's more the game that comes up on Saturday [against Leeds].

"We had an early game against West Ham and then a late game against Zenit. We had a day off and good training session today.

"The problem is the next match because we will lose time to sleep and arrive back very early on Thursday. Leeds are an intense team who have prepared to face this match."

Asked about Kovacic's positive coronavirus test, Tuchel said: "I was aware this can happen and I'm not happy, but this is the reality right now. We have to be ready to adapt.

"There is a huge chance that we make changes against Zenit."

Kovacic had previously been recovering from a hamstring injury sustained in training, with the Croatia international's most recent appearance coming on October 26.

Kante has missed Chelsea's previous three games with an ailment sustained in the 4-0 win over Juventus, meanwhile, and Tuchel has no intention of rushing the France international back.

"I'm careful with bringing him back. I have quite an open relationship with him," Tuchel said. "He's a careful person and takes care of his body.

"I agree he's a superman who makes the difference and everyone in the world wants him on their team. It's not related to age or overload – it was an accident in the match."

Chelsea have conceded just three goals in 12 Champions League games under Thomas Tuchel

The absence of Kovacic, Kante and Jorginho may see Saul come into the side for what would be just his eighth appearance in all competitions since joining on loan in September.

Saul has struggled to impose himself so far, with his passing accuracy of 77.98 per cent the lowest of any Chelsea central midfielder, but Tuchel is hoping to see more from the Atletico Madrid loanee.

"He's one of the guys that needs minutes. He is always in our thoughts and we think about every training session and form," he said.

"I think he struggled a bit with the intensity and he's a bit unlucky he played against Aston Villa, Southampton and Watford, all three were high-intensity games, above average.

"We try to encourage him because he grows into Chelsea more and more. We won't stop encouraging him and trying to bring out the best. It's possible he will improve tomorrow."

Chelsea have won only three of their past six games in all competitions, most recently losing 3-2 to West Ham on Saturday as they lost control of the Premier League title race.

The Blues are already assured of a place in the last 16 of the Champions League ahead of their clash with Zenit, but they are battling with Juve for top spot.

However, Tuchel is not panicking as his side aim to become the second English team after Manchester United (in both 2003-04 and 2010-11) to win four straight games in the competition without conceding.

"We dropped some points against Man United, found a solution to win at Watford, and we lost with a freak shot against West Ham," Tuchel said.

"We had this before. We lost games and we don't like it. But we're not worried. We have a certain anger and ambition when you don't win.

"Everyone hates to lose but it happens. But when we analysed the game, the data, watched the game back, I don't think we need to ask big questions."

The only previous meeting between Zenit and Chelsea came in September, the Blues recording a 1-0 victory at Stamford Bridge through Romelu Lukaku's second-half winner.