Yerry Mina is set to be sidelined for up to the next 10 weeks.
Mina pulled up midway through the first half of Everton’s 3-1 defeat at Newcastle United on Tuesday night with a suspected quadricep problem.
The Colombia international’s truncated outing at St James’ Park was only his second start for the Blues since early December before suffering the setback.
Frank Lampard confirmed on Friday that Mina now faces up to two months due to this latest injury, which leaves his side light on centre-back options.
Everton have also lost Ben Godfrey as a result of a hamstring tear sustained during last weekend’s comfortable FA Cup fourth round win over Brentford.
“Yerry Mina has an unfortunate injury that will be probably between eight and 10 weeks [out],” said Lampard.
“It’s a strong injury around his quad. It’s disappointing for us all.
“In the week I’ve been here I’ve realised he’s a big leader in the group and a top player for us, obviously, so it’s a big miss.”
He added: “I was aware on the night of how serious it was and our worst fears were confirmed.
“Yerry’s a big personality here, he’s a positive man who we’ll miss.”
“Demarai Gray won’t be fit for Leeds but hopefully it’s just a minor knock around his hip and we’re hoping he’ll be fit for next week with some luck.
“Fabian Delph is not ready yet. He’ll be, hopefully, a couple of weeks away. He’s making good progress – I’ve actually just spoken to him.”
Demarai Gray will also be unavailable for Saturday’s visit of Leeds United through a hip injury picked up in the midweek reversal to Eddie Howe’s side.