The Baggies appeared set for a comfortable afternoon after first-half goals from James Morrison and Zoltan Gera had put them in control, but they conceded twice to ensure a nervy finish.
And, despite seeing his side seal a fourth home league win of the season through Youssouf Mulumbu, the Scot lamented the defensive display against the Rs.
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"From 2-0 up I thought we became a bit too open. Defensively it wasn’t a performance you would associate with us.
"Attacking-wise we played some fantastic football going forward and created a lot of chances in the game. It took a little bit longer to see the game out than it should have done."
The former Liverpool assistant boss was quick to praise his side's attacking display, however, particularly in the build-up to their third goal.
"It started on one side of the pitch and we worked it across to the other," he continued.
"I’m not sure what the right-back Gonzo was doing up there at that stage of the game. He was there and he played a great little one-two with James Morrison and produced a great cut-back for Mulumbu.
"That was a good goal but even from there I can say I thought we were a little bit sloppy defensively today, which is not like us"