La Roja struggled to break down their opponents for long spells, with Xavi admitting that this side were well below bar despite putting qualification firmly within their grasp.
He told Telecinco: "We weren't effective enough with the final pass but in the end we got the win and it's a very important step towards the World Cup.
"Alvaro's goal gave us some breathing room and at the end the Belarus goal gave us a bit of a shock but it turned out fine.
"It's tough when teams shut up shop at the back with all 10 players in their half."
Del Bosque's men now need just one point from their final qualifier against Georgia to earn a trip to Brazil next summer, which would see La Roja arrive as defending champions.