Roberto Martinez confirmed on Thursday that Jagielka has suffered a hamstring injury and is certain to miss the weekend game at Goodison as well as England's friendly with Denmark at Wembley on Wednesday.
It is a blow that further weakens an already threadbare squad at a crucial stage of the season and his absence will be especially felt against the Hammers, who will not doubt fling a succession of high balls into the Blues penalty area for Andy Carroll and Kevin Nolan to feed off.
However, Jags' absence means we will get to see the enormous potential of John Stones, who is almost certainly set to deputise.
The 19-year-old looked calm and composed when he came into the team just after Christmas, including a solid display away at Stoke City on New Years's Day, the true litmus test of any Premier League centre half.
His last appearance was admittedly one to forget - though he wasn't alone in playing poorly in the 4-0 drubbing at Anfield last month.
That, though, was at right back, rather than his preferred position of in the centre, and he should thrive alongside the experienced Sylvian Distin.
Losing Jags is definitely a blow and we can only hope he is fit in time for the Arsenal game next week, a match that has taken on even greater significance after recent poor results.
But in Stones we seem to have an able deputy and one who looks to have the potential to go to the very top, meaning this latest injury carries with it a very exciting silver lining.