Spain boss Vicente del Bosque admitted his side had struggled more than he would have liked to seal their place in next year's European Championships after sealing qualification with a 4-0 win over Luxembourg on Friday.
Santi Cazorla and Paco Alcacer each scored twice to give the European champions an unassailable five-point lead over Slovakia and Ukraine with just one game to play in Group C.
Spain have won eight of their nine qualifiers and haven't conceded a goal in the seven games since they were beaten 2-1 away at Slovakia exactly a year ago.
However, Del Bosque admitted that Spain had failed to reach the heights that made them champions in each of the last two European Championships.
"We have had difficult moments, but fortunately we reacted well as a team. Generally we have defended pretty well and I think today we also created a lot of chances and should have scored some more," said the former Real Madrid coach.
"The defeat in Slovakia took its toll on the rest of the campaign. In terms of how we are playing and the development of the team I think we are pretty well advanced.
"I wouldn't say we have 11 players that will always play, but we have an idea of how we want to play and everyone is going in the right direction."
However, victory and qualification were overshadowed by injuries to David Silva and Alvaro Morata and the repeated jeers towards Gerard Pique by the home fans.
Initial reports in the Spanish media suggested Manchester City's Silva has suffered a right ankle sprain, while Juventus striker Morata has been taken to hospital for tests on what is feared could be a broken fibula.
"The worst part of the game was the two injuries," added Del Bosque.
"They were two unfortunate clashes, but until we have the medical reports we don't know how serious they are."
Pique was once again whistled despite the outspoken requests of Del Bosque and his teammates to support the Barca centre-back.
The former Manchester United defender has been targeted since mocking Real Madrid during Barcelona's treble-winning celebrations at the end of last season.
"I am tired of the subject, but I also recognise that we have to thank the people of La Rioja and Logrono who were supportive and showed a lot of warmth towards the national team," said the coach.
"I am left with that much more than the people who have whistled."
The absence of Chelsea striker Diego Costa through suspension opened the door for Alcacer to be called up despite his poor start to the season with Valencia.
And he again proved more adept at international level than Costa, who has scored just once in nine games for his adopted country, with two goals after replacing Morata.
"Strikers live off scoring goals and that is what I offer the team," he said after taking his tally to six goals in nine games for Spain.
"Here things seem to go well for me. That doesn't mean I can let my guard down, though, I have to keep trying so I can come back more often."
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