I'm sure Lampard has had a look at the teams performances this season and decided, with good reason, that the biggest problems we have are, the disorganization in defence and the lack of quality in midfield. He probably has decided that defensive solidity can be improved on the training ground, but, that the lack of quality in midfield could only be addressed by bringing in some players with natural ability.
It's not as if he had unlimited time or money to make his signings and I'm sure he's prioritized and brought in the players he thought we most urgently needed. I'm sure, provided we stay up,that we will see big changes in the summer, at least one big sale and some solid purchases.