The Toffees' captain believes his side will need another fruitful summer if they are to continue their challenge for a top four berth next season under Roberto Martinez
Gareth Barry, Gerard Deulofeu and Romelu Lukaku are all set to return to their parent clubs at the end of the season after becoming integral parts of Everton's success this season.
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And Jagielka, who returned to first-team action on Saturday after over two months out with injury, believes the Spaniard's summer transfer dealings will play a decisive role in the club's fortunes next season.
"We'd need to replace Gareth [Barry], Rom [Lukaku] and Gerard [Deulofeu], if they move on. We can't flourish otherwise," he told the Liverpool Echo.
"It's difficult to bring in players of the same quality but that's the manager's job.
"We'll be the first to think the lads if they decide either not to extend their loans or against coming back next season. We'll thank them for what they've done because we knew that was the situation when we signed them; that we maybe only had them for a season.
"So now we need to unearth a few more gems or twist people's arms to get them to stay next season.
"I think with the squad we've got, we wouldn't accept performing well in one competition and being disastrous in the other one. That's the type of lads we are.
"And with the right people coming in over the summer, and a bit more depth to the squad, it's an exciting time for us."