Blessed translates the same adjective used in the Beatitudes (see comment at 5.3).

So doing is translated “doing this” by Good News Translation. The full meaning is “doing the work that his master has assigned him.” New English Bible translates “at his task” (similarly Die Bibel im heutigen Deutsch), and Jerusalem Bible has “at this employment.” Barclay translates by a verb construction: “engaged on that very task.”

The verse may have to be restructured slightly; for example, “Things will be very good for that servant if the master comes and finds him doing that work (he assigned him).”

Quoted with permission from Newman, Barclay M. and Stine, Philip C. A Handbook on the Gospel of Matthew. (UBS Handbook Series). New York: UBS, 1988. For this and other handbooks for translators see here .