I have no problems with a midsummer DP arrival for the right guy. We basically played with 2 DPs last year, notwithstanding Medina's very sporadic contributions. Bonus that it guarantees that my hobbyhorse club stat (never having a season in which at least one DP does not miss 10 games) gets another year, and the club could still very well finish the season with its best DP contingent in terms of concurrent contributions.
And if we can transfer Medina this winter to remove any fire sale drama that's all the better.
Not to mention that we are likely losing our second striker mid-season this year, Taty. Swapping in Mertens would be awesome.
I'm also just going to drop use of current roster rules, things might look drastically different under the new CBA.