Jagielka, 28, looked back to his best as he returned to the starting line-up in last weekend's dramatic FA Cup fourth-round replay win at Chelsea.
Jagielka played the full 120 minutes at Stamford Bridge as Everton won on penalties, ending a frustrating sequence which began when he suffered a thigh strain at Manchester City on December 20.
Former Sheffield United player Jagielka missed four of the subsequent five games and then spent the next five games after that on the bench, getting onto the field just twice.
Moyes said: "He wasn't right to play. He went back to see the specialist who said he didn't think the thigh strain had quite healed. We could only use him against Bolton in an emergency.
"I put him on the bench in case of emergency but he was not really to be used in any other capacity. He is better now but has carried a lot of injuries this year and we are concerned about it. He has never really shaken things off. That is why he has maybe not quite as consistent a run as we would like.''
Prior to his injury Jagielka - who missed the first five months of last season with a serious knee injury - had been an ever present in the Premier League this term. Moyes does not feel he can understate the importance of a fully-fit Jagielka to his team, although he does think he has adequate cover in central defence.
Moyes added: "He is a really big player for us and his performance last week was immense. We miss him when we don't have him but we have got good competition at centre-half with Jonny Heitinga and Sylvain Distin.''