Portsmouth face second-placed Leicester on Saturday and then travel to fellow promotion-chasers Wolves four days later - two games which could decide if they are destined for automatic promotion.
Merson said: "The next three matches are very big games for us but if we do well then we could put ourselves in a very good position.
"We didn't play well against Preston, especially in the second half, but the important thing was we got the result.
"You don't get any extra points for entertaining people and the most important thing was that we claimed another victory."