Mendez-Laing was a surprise inclusion on the bench against Nottingham Forest at the City Ground on Saturday, coming on as a 87th-minute substitute during the 2-1 defeat.
The 20-year-old, a summer signing from Wolves, was shipped out of the club by Ferguson in November along with striker Tyrone Barnett for his involvement in a city centre disturbance which saw both players cautioned by police.
He was sent out on loan to League One strugglers Portsmouth but returned to London Road last week.
Ferguson told the Evening Telegraph: "I don't bear grudges. Mendez-Laing is back at the club and it's now up to me and the coaching staff to get the best out of him as a player.
"He clearly has ability, but he must also now knuckle down and do all he can to help this team."
Mendez-Laing told Peterborough fans via Twitter on Saturday night: "It felt good to put on a Posh shirt again."