Zlatan Ibrahimovic was leading United's attack last campaign as the former Barcelona and Paris Saint-Germain striker scored 28 goals in all competitions. United went on to win the EFL Cup and Europa League while finishing sixth in the league table in Mourinho's first season at Old Trafford.
The Swede suffered a knee injury in April and United released him after his contract expired, only to re-sign him in August. He is still recovering on the sidelines as he has not made an appearance since his injury against Anderlecht last season.
In his absence, United signed Lukaku from Everton in the summer transfer window. The Belgium international has been in fine form for the Red Devils, scoring 11 goals in all competitions.
Meanwhile, Mourinho has won league titles at Porto, Chelsea, Inter Milan and Real Madrid in his second season. Shearer believes his past record, coupled with the addition of Lukaku and Nemanja Matic gives the 20-time English champions the edge over Manchester City in the title race.
"Well, he [Mourinho] normally does well in his second season. He won two trophies last season. I just think he seems very confident and content with the team he has. Got the players he wanted and a striker who is in his prime of his career, who should get 25 to 30 goals this season," Shearer told IBTimes UK.
After United's goalless draw against Liverpool, the Old Trafford club are second in the table with 20 points after eight games. Mourinho's side are two points behind league leaders City, who have already registered victories over the Reds and champions Chelsea.