The challenge that Cullen as to overcome is that he's not being used as Rashford et al are used. He is in a RM role where he has significant defensive responsibility; almost playing box to box except that he rarely gets into the other team's box unless the play develops down the left side or there's a set piece, or rarely when the play has developed down the right to the side of the box and it is rolled back/inside to Cullen to then cut inside and have a go. And as a box-to-box player when he gets into the box he's had to run the length of the field to get there, so he's not going to participate in quick counter attacks unless the ball is won high up the field.
With the way the team has typically been selected - Grimes/Fulton/Walsh (now somebody else)/Pato in midfield - Lowe has typically been started in preference to Cullen as the #9. Sheehan has to decide that Cullen should be the preferred #9 and use him as high up the field as possible. The temptation is going to be to use Cullen to replace the injured Walsh, which gets Cullen on the field but too far from his point of best use.
Sheehan is also going to be tempted to use Lowe in a RM/LM role and play Yates at #9 leaving Cullen out; which is the height of idiocy IMHO. Such a move is not consistent with performance, abilities and player best use.
Until other players start scoring, the best finisher we have is still Cullen.
This season:
Cullen: 25 appearances (starting/sub), 1,130 minutes, 4 goals, 3 assists, 283 minutes per goal ... from midfield (RW/RM)
Yates: 26 appearances, 1,630 minutes, 6 goals, 0 assists, 272 minutes per goal ... always #9
Lowe: 21 appearances, 1,718 minutes, 5 goals, 2 assists, 344 minutes per goal ... some #9, some LW/M combo.
As you can see from these numbers, Cullen has been playing the deeper of the 3, with arguably the best return in terms of goals and assists. WTF not play him at #9 - his more natural position.
Note, we've been playing mostly 4-2-3-1 under Sheehan, but this is not the same 4-2-3-1 as Rogers who used Sinclair and Dyer as wingers. We have not been using "wingers".