Spain won the match in Palma de Mallorca 2-1 thanks to second-half goals from captain Xavi and substitute Alvaro Negredo but again toiled when faced with a side at times deploying all 10 outfield players behind the ball.
The hosts were also given a late scare when Sergei Kornilenko pulled one back for the visitors in the 89th minute but were able to ride out the final few minutes unscathed.
The laboured victory lifted the Spanish three points clear of second-placed France with one match to play in Group I and a point against Georgia in Albacete on Tuesday will secure a berth at the finals in Brazil next year.
However, Del Bosque suggested some improvements will be needed if Spain are to become the third nation to retain their world title after Italy in 1938 and Brazil in 1962.
"Perhaps we didn't find the means of opening up the pitch," Del Bosque told reporters.
"We are clear about what to do against such a massed and intense defensive line but at times we are not effective," the former Real Madrid coach added.
"We were lacking penetration down the wings and we need to look to take players on more, shoot more at goal and be a bit more aggressive in attack.
"We are analysing what happened today and we will have time from tomorrow to think about what to do on Tuesday."
Del Bosque sprung a surprise in his starting lineup by giving a debut to Swansea City forward Michu, who was only brought into the squad on Sunday as a replacement for the injured David Villa.
Michu worked hard in a central striking role and had a couple of half-chances before he was replaced by Negredo and Del Bosque suggested he would have another chance to show he deserves to be in the squad for the trip to Brazil next year.
"Of course the match wasn't easy for him," Del Bosque said.
"I told him at the end that I value his efforts and he will have another opportunity to prove his worth."
Negredo improved his chances of securing the centre forward slot ahead of the likes of Villa, Michu, Roberto Soldado, Fernando Torres and possibly Brazil-born Diego Costa with his well-taken goal in the 78th minute.
The Manchester City striker timed his run into the penalty area perfectly before flinging himself full length to head a Sergio Ramos cross low into the corner of the net.
"The competition for places means you don't relax for a moment and keep working as hard as possible," Negredo said in an interview with radio broadcaster Cadena Ser.
"We all want to be there in Brazil next year and you just have to try and win your place little by little," he added.
- Sports & Recreation
- Alvaro Negredo
- Del Bosque
- Sergei Kornilenko