Only Loic Remy has arrived on-loan from QPR so far this summer.
"It's beginning to be a bit of a concern, the gap they're opening up to the rest of the Premier League.
"Financial Fair Play was meant to even things out a little bit, but it doesn't seem to have had any impact at all.
"We can only try to improve with what we have. I do think we will do some business before the window shuts, so perhaps it's a bit more lopsided as things stand.
"We have invested in the team and believe there is good depth to the squad, but I think the offensive players turn draws into wins and are very important to the Premier League."
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