The 25-year-old – also a target for Liverpool – is expected to leave the Lisbon giants in the New Year after struggling in the Portuguese capital since his 2011 move from Sevilla.
Alan Pardew is ready to dip into the transfer market next month after enduring a woeful summer of activity – in which Loic Remy was the only notable arrival – and is keen to tie up the two time Spanish international.
A long-term replacement for Jonas Gutierrez is on the ex-Southampton coach’s radar, and he sees the £5m-rated Capel as player who can succeed the Argentine.
A number of clubs across Europe are believed to be tracking the talented wide-man, but the Magpies are hopeful that the lure of first-team football at St James’ Park may be enough to tilt the scales in their favour.
Meanwhile, Pardew’s transfer window priority remains securing a new striker.
Support for Remy is needed thanks to Papiss Cisse’s dip in form, with Bafetimbi Gomis – who came close to joining the ‘Toon’ from Lyon a matter of months ago an option.