Among defined contribution plans, the study said, a shift in resources should be expected over the next two years toward benefit adequacy and retirement readiness over the next two years.

Retirement benefit cost will still be a major concern for employers, but to a lesser extent during this period. In fact, the study said, it is expected that in two years, retirement readiness will be roughly tied with regulatory compliance as the third most important retirement plan risk.

The move to defined contribution plans is fueling concerns over retirement readiness, with 58 percent of sponsors that have never offered a defined benefit plan indicating that retirement readiness is their top risk, in contrast to 39 percent of all respondents.

"Not surprisingly, retirement benefit adequacy and the financial fitness of their workers are growing concerns among employers," Dave Suchsland, senior retirement consultant at Willis Towers Watson, said in a statement. "This is particularly true among employers that offer only a DC plan. In fact, workers' inability to retire in a timely fashion was identified as the top risk for nearly six in 10 of these plan sponsors. The ongoing shift to DC plans is now prompting employers to prioritize resources that promote retirement readiness."

While currently a majority of defined contribution plan sponsors are devoting their top investment resources primarily to monitoring investment fees (74 percent) and manager performance (61 percent), employers are planning a greater focus on benefit adequacy and monitoring participant behavior.

Survey respondents indicated that the percentage of defined contribution plan sponsors that prioritize benefit adequacy will more than double in the next two years, from 18 percent to 38 percent.