Scotland Under-21s are getting an improved Ben Doak after a carefully considered break for the Liverpool winger, head coach Scot Gemmill said.
"He listened to his body and knew he needed a significant rest over the summer," Gemmill explained. "He's gone back to Liverpool fully focused, fully fit, stronger, faster, more determined than ever."
Doak has started the season in all three of Liverpool's Premier League matchday squads, coming off the bench in the opening weekend draw at Chelsea.
"It's really exciting to see a player have that ambition, have that mentality, and it's great to see it being rewarded," Gemmill said.
"He's definitely a very exciting player. The player always has to go and prove it and really keep pushing. The big part of the jigsaw is the attitude.
"Ben has put himself in a position now where he really has got an opportunity to play at the highest level, so everybody needs to do everything they can to support him and help him on that pathway.
"I think back to watching him play for the Scotland Under-16s and he wasn't convincing me in terms of whether he was really ready to push on. When things didn't quite go his way, you could see how emotional he was, how angry he was.
"But he really has fast-tracked in the last couple of years now where he seems to be able to focus in on his performance and the biggest thing is that he's influencing games against top opposition."
Wing-backs Josh Doig and Max Johnston also return after missing the June friendlies, but Tommy Conway, Lewis Fiorini and Calvin Ramsay are absent through injury.
Gemmill is sure the Spanish side will contain "future superstars" but thinks his players can cause an upset in Jaen.
"We played Holland four years ago and nobody expected us to beat such a high-ranked team with players like [Frenkie] De Jong, but we beat them," he said.
"Two years after that, we did exactly the same in Croatia. Every one of that Croatian back four has been transferred for over 50 million euros.
"I have the belief we can go and do the same in Spain and, by the time kick-off comes, the players will have the belief too."
Scotland Under-21 squad
Goalkeepers: Cieran Slicker (Ipswich Town), Jude Smith (Newcastle United), Jack Newman (Dundee United).
Defenders: Ibane Bowat (TSV Hartberg - on loan from Fulham), Matthew Anderson (Admira Wacker Modling - on loan from Celtic), Josh Doig (Hellas Verona), Max Johnston (Sturm Graz), Leon King (Rangers), Ben McPherson (Queen's Park - on loan from Celtic), Liam Morrison (Wigan Athletic - on loan from Bayern Munich), Lewis Neilson (Partick Thistle - on loan from Heart of Midlothian), Jeremiah Mullen (Leeds United).
Midfielders: Azeem Abdulai (Swansea City), Connor Barron (Aberdeen), Lyall Cameron (Dundee), Ryan Duncan (Aberdeen), Alex Lowry (Heart of Midlothian - on loan from Rangers), Archie Meekison (Dundee United), Ben Summers (Dunfermline Athletic - on loan from Celtic), Josh Mulligan (Dundee).
Forwards: Kieron Bowie (Northampton Town - on loan from Fulham), Ben Doak (Liverpool), Dire Mebude (Westerlo).