Um. No. Javier Mascherano is by far the second best central defender at Barcelona and height alone shouldn’t be a factor. Age is a consideration with any position but that doesn’t simply mean shipping Mascherano off, especially when he could still play as a defensive midfielder. Right now, Mascherano still has the necessary recovery pace, positional sense etc to perform well and his partnership with Pique is very good.
Yes, Barcelona need a new defender if they aren’t going to trust Marc Bartra to progress, and if they keep Vermaelen and Mathieu as backup. If Marquinhos decides he wants to leave then that makes such a move easier but paying an eye-watering fee for young talent that may just end up not making the grade(not necessarily Marquinhos but someone like Laporte) isn’t a sound decision.