The 33-year-old returned to the fold for Everton on Saturday.
A hamstring injury had kept the 33-year-old defender out of the Toffees' full pre-season calendar.
But the veteran centre-back announced his return in Saturday's 1-0 friendly defeat by Espanyol.
Despite the result, Koeman, who only replaced Roberto Martinez this summer, is delighted to have Jagielka back and said he's happy with his performance.
“It was very good to have Phil Jagielka back in the squad," he told the club's official website.
“If you miss two or three weeks of pre-season then it’s difficult but experienced players know the game and he showed a good mentality and performance today.”
It is indeed a timely return for Jagielka as Everton now gear up to host Tottenham Hotspur in Saturday's Premier League curtain raiser.
Koeman might well need Jagielka on the day because, if rumours are to be believed, then there might be a John Stones-shaped hole in the heart of their defence.
The 22-year-old defender started the defeat by Espanyol, but he remains the subject of intense speculation as stories of a £50 million move to Manchester City continue to circulate.